Etymology
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chin (n.)

"lower extremity of the face below the mouth," Old English cin, cinn "chin," a general Germanic word (compare Old Saxon and Old High German kinni; Old Norse kinn; German Kinn "chin;" Gothic kinnus "cheek"), from PIE root *genu- (2), probably originally "jaw, jawbone," but also forming words for "chin, cheek."

The West Germanic words generally mean "chin," but there are traces of earlier use as "jaw," such as Old English cinbane "jawbone," and the words for "cheek," "chin," and "jaw" naturally overlap and interchange; compare cheek (n.), which originally meant "jaw," and Latin maxilla, which gave Italian mascella "jaw," but Spanish mejilla "cheek."

To take it on the chin "be hit hard" in a figurative sense (sometimes suggesting "ability to withstand punishment"), is from 1924, an image from pugilism. To keep (one's) chin up "remain optimistic amid adversity" is from 1913, though the image itself is older.

I discovered the other day another simple means of producing cheerfulness—raise the chin—with the chin up, the whole mental attitude is changed. If you feel a bit blue or discouraged, just raise your chin, and you will find that things look different; whereas the mere appearance of a man with his chin down suggests that he is disconsolate. [National Magazine, November 1906] 
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chin (v.)

1590s, "to press (affectionately) chin to chin," from chin (n.). Meaning "to bring to the chin" (of a fiddle) is from 1869. Slang meaning "to talk, gossip" is from 1883, American English. Related: Chinned; chinning. Athletic sense of "raise one's chin over" (a raised bar, for exercise) is from 1880s.

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chin-up (n.)

also chinup, type of exercise, 1940, from chin (v.) + up (adv.). Earlier it was called chinning the bar and under names such as this is described by 1883.

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*genu- (2)

Proto-Indo-European root, probably originally "jaw, jawbone," but also forming words for "chin, cheek." It forms all or part of: chin; Compsognathus; gnathic; gnatho-.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit hanuh "jaw," Avestan zanu- "chin;" Armenian cnawt "jawbone, cheek;" Lithuanian žándas "jawbone;" Greek genus "chin, lower jaw," geneion "chin;" Old Irish gin "mouth," Welsh gen "jawbone, chin;" Old English cin, "chin."

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cheek (n.)

"either of the two fleshy sides of the face below the eyes," Old English ceace, cece "jaw, jawbone," in late Old English also "the fleshy wall of the mouth," of uncertain origin, from Proto-Germanic *kaukon (source also of Middle Low German kake "jaw, jawbone," Middle Dutch kake "jaw," Dutch kaak), not found outside West Germanic, probably a substratum word.

Words for "cheek," "jaw," and "chin" tend to run together in IE languages (compare PIE *genw-, source of Greek genus "jaw, cheek," geneion "chin," and English chin); Aristotle considered the chin as the front of the "jaws" and the cheeks as the back of them. The other Old English word for "cheek" was ceafl (see jowl (n.1)).

A thousand men he [Samson] slow eek with his hond, And had no wepen but an asses cheek. [Chaucer, "Monk's Tale"]

In reference to the buttocks from c. 1600. Sense of "brazen insolence" is from 1840, perhaps from a notion akin to that which led to jaw "insolent speech," mouth off, etc. To turn the other cheek is an allusion to Matthew v.39 and Luke vi.29. Cheek-by-jowl "with cheeks close together," hence "in intimate contact" is from 1570s; earlier in same sense was cheek-by-cheek (early 14c.). In ballroom dancing, cheek-to-cheek is from 1919 (earlier it was a measurement of apples). 

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ch 

digraph used in Old French for the "tsh" sound. In some French dialects, including that of Paris (but not that of Picardy), Latin ca- became French "tsha." This was introduced to English after the Norman Conquest, in words borrowed from Old French such as chaste, charity, chief (adj.). Under French influence, -ch- also was inserted into Anglo-Saxon words that had the same sound (such as bleach, chest, church) which in Old English still was written with a simple -c-, and into those that had formerly been spelled with a -c- and pronounced "k" such as chin and much.

As French evolved, the "t" sound dropped out of -ch-, so in later loan-words from French -ch- has only the sound "sh-" (chauffeur, machine (n.), chivalry, etc.).

It turns up as well in words from classical languages (chaos, echo, etc.). Most uses of -ch- in Roman Latin were in words from Greek, which in Greek would be pronounced correctly as /k/ + /h/, as in modern blockhead, but most Romans would have said merely /k/, and this was the regular pronunciation in English. Before c. 1500 such words were regularly spelled with a -c- (Crist, cronicle, scoole), but Modern English has preserved or restored the etymological spelling in most of them (chemical, chorus, monarch). 

Sometimes ch- is written to keep -c- hard before a front vowel, as still in modern Italian. In some languages (Welsh, Spanish, Czech) ch- can be treated as a separate letter and words in it are alphabetized after -c- (or, in Czech and Slovak, after -h-). The sound also is heard in words from more distant languages (as in cheetah, chintz), and the digraph also is used to represent the sound in Scottish loch.

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goatee (n.)
"pointed tuft of beard on the chin of a shaven face," 1844 (as goaty; current spelling by 1847), from goaty (adj.). So called from its resemblance to a male goat's chin hairs.
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chinned (adj.)
"having a chin or chins" (of a certain kind or number), used in combinations from c. 1600.
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chuck (n.2)

"slight blow under the chin," 1610s, from chuck (v.1). Meaning "a toss, a throw" is from 1862.

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