- The English word "body" is a bit of a mystery.

It's descended from the old English "bodig," but beyond that, no one knows where it came from, and it has no confirmed existing cognates.

Other Indo-European languages tend to use some derivation of the Proto-Indo-European root "kwrep," as in the French "corps," in Spanish, "cuerpo," or the root "leip," which meant to stick or adhere, and gave us the German "Leib," Swedish "liv," and English "life."

(upbeat cheerful music) As we've covered before, Proto-Indo-European is a hypothetical language spoken by people some scholars suggest lived near the Black Sea some 8,000 years ago.

It's the ancestor of hundreds of existing languages, including Spanish, German, Sanskrit, Persian, Swedish, Italian, Armenian, and of course English.

Comparative linguists have been able to deduce a fair bit about Proto-Indo-European culture by discovering common word origins.

For example, since several languages as disparate as Sanskrit, Greek and Dutch have similar words for wheel, it stands to reason that the Proto Indo Europeans had wheeled vehicles, and linguists have reconstructed a PIE root, "k-w-el."

However, we don't need comparative linguistics to tell us that the Proto-Indo-Europeans had bodies, body parts would've probably been one of the first things that ancient peoples gave names to.

So not only do we find hundreds of cognates and permutations, but also widespread use in idioms, slang and other figures of speech.

Since our bodies have been with us literally from the beginning, we can learn a lot about language by investigating their etymologies.

I'm Dr. Erica Brozovsky, and this is "Otherwords."

(upbeat quirky music) - Otherwords.

- Let's start at the top.

The top of the body, that is.

Head comes from the old English "heafod," which in turn originated from the PIE root "kaput."

Dozens of English words are descended from the Latin version, including "captain," "capital," "decapitate," "chief," "chapter, and "cattle."

(groovy upbeat music) You might be wondering how one gets from "kaput" to "heafod," since they really don't sound anything alike.

This question is actually behind one of the greatest leaps in the study of linguistics.

In the 19th century, some linguists began to notice a pattern between Latin and Germanic languages.

Words that had a 'p' a sound in Latin languages, instead had a 'f' sound in Germanic languages.

Compare the Italian "padre" and "pesce" to the English "father" and "fish."

Several other sound patterns were also identified, like the Latin 'k,' and the Germanic 'h,' which is how the Latin "canis" became the English "hound."

And why Latin question words like "quis," "quid" and "quo" start with a 'qu',' while English question words like "what," "where" and "when" start with a 'wh.'

These patterns were detailed in the book "Deutsche Grammatik" by Jacob Grimm of the Grimm Brothers fame.

Like a 19th century Tolkien, Grimm was obsessed with folklore and linguistics, and his observation, which stemmed in part from the work of other linguists, came to be known as "Grimm's law."

It led to the belief that these seemingly unrelated languages had to share a common ancestor, and that sometime after Proto-Germanic split off, it underwent systematic sound changes across its entire lexicon.

This is how the 'k' and 'p' of kaput slowly became the 'h' and 'f' of "heafod."

To bring it back to body parts, Grimm's law also applies to the English foot, which comes from the PIE "ped," again, 'p' becomes 'f.'

And it has many English cognates like "pedal," "pedestrian" and "octopus."

Likewise, "finger" may have descended from the PIE "penkwe," which meant "five," as in "pentagon" and "quintet."

Grimm's law works in chains of phonemes, with one sound change pulling or pushing the others in the chain along with it.

For example, in this chain, the 'd' sound becomes 't,' and the 't' sound becomes 'th.'

So the PIE root "dent" becomes the Proto-Germanic "tanthu."

And finally, the English tooth, which means that tooth is cognate with, not surprisingly, "dentist," "denture" and "indent."

While we're talking about teeth and feet, why is it that the plurals of these nouns are so irregular?

Why not "tooths" and "foots"?

(idle quirky music) This is due to another sound shift known as i-mutation.

At the time, the usual method of making plurals was not that different from modern English, adding 'iz' to the end of a word, so the plural of "fot" might have been "fotiz."

But something odd can happen when we have a low back vowel, '-o' followed by a front high vowel, 'i.'

It's a long distance for your mouth to travel, '-o-i.'

So to save effort, the first vowel gets shifted up and forward closer to the second.

It's kind of like how if you say this sentence casually, it probably sounds less like, "How are you doing?," and more like, "How're you doing?"

So the plural for foot probably sounded less like "fotiz" and more like "fetiz."

Now that the first syllable already indicates plurality, the second syllable becomes redundant, so it eventually dropped away, (glass breaking) leaving us with "foot" and "feet."

The same thing happened to "tooth" and "teeth," "man" and "men," "mouse" and "mice," and many other English nouns with irregular plurals.

Several body parts seem to have been named after their appearance or structure.

"Knee," "elbow," "wrist" and "ankle" all have PIE origins that meant "bend" or "angle."

(idle quirky music continues) Thumb is from the root "teue," which meant "to swell," making it cognate with "tumor."

And hair may come from the PIE root "ghers," which meant "to stand out or bristle," also the root for the word "horror."

"Arm" comes from the PIE "ar," which meant "to join or fit together," probably a reference to all its component parts.

Because arms are so key in human production, and destruction, it became the root for many words related to craftsmanship and warfare.

"Hand" and "leg" both came about after Proto-Germanic split from PIE, and their origins are uncertain.

However, "palm" comes from the PIE "pele," which meant "flat or spread," and is cognate with "plain," "field" and "floor."

"Eye" is derived from the PIE "okw," "to see," and is cognate with "ocular, "optic," and "window," which literally means "wind eye."

I like that.

(upbeat quirky music) If we peel back the skin, which comes from "sek," "to cut," we find another victim of Grimm's law, with the 'k' and 'd' of "kerd" becoming the 'h' and 't' of "heart," making it cognate with "cardiac" and "courage."

Ancient people believed that the heart, and not the brain, was the center of mental faculties, so it's also related to words like "creed," "credible" and "record."

That sense still exists today when we say we "know something by heart."

"Lung" comes from the PIE root "legwh," meaning "not heavy."

This may be because when boiling organs, as one does, the lungs would float to the top of the pot.

Brain may have come from the PIE "mregh," meaning "skull," or "bhragno," meaning "something broken," maybe because it looked split down the middle, or maybe because that's what you should aim at in an ax battle.

These words have been with us for so long, it's no wonder we found countless creative ways to use them.

We can use anthropomorphism to clarify the structure of inanimate objects, as in "head of a ship," "foot of the bed" or "shoulder of the road."

Metonymy is the usage of a thing to refer to an associated concept.

If you "give someone a hand," that can mean assistance or applause.

If someone has "heart," they possess courage or kindness.

"Legs" can mean endurance, "stomach" can mean tolerance, and "skin" can mean something of value that's at risk.

That's not to mention the literally hundreds of idioms that involve body parts, from "break a leg" to "eat your heart out."

How many can you think of off the top of your head?

The parts of your body are with you hopefully from birth to death.

They're how we perceive and interact with external objects.

They're a filter and point of reference for understanding our world.

So naturally, their linguistic labels bleed into all other areas of life.

It's a lot to wrap your brain around, but if there's one thing this audience has,