leer v.

Proto-Indo-European root meaning "to hear."

It forms all or part of: ablaut; Cleon; Clio; Damocles; Hercules; leer; list (v.2) "hear, harken;" listen; loud; Mstislav; Pericles; Slav; slave; Slavic; Slovene; Sophocles; Themistocles; umlaut; Wenceslas; Yugoslav.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit srnoti "hears," srosati "hears, obeys," srutah "heard of, celebrated;" Avestan sraothra "ear;" Middle Persian srod "hearing, sound;" Greek klyo "hear, be called," klytos "heard of, celebrated," kleos "report, rumor, fame glory," kleio "make famous;" Latin cluere "to hear oneself called, be spoken of," inclutus "renowned, famous;" Armenian lu "known;" Lithuanian klausau, klausyti "to hear," šlovė "splendor, honor;" Old Church Slavonic slusati "to hear," slava "fame, glory," slovo "word;" Old Irish ro-clui-nethar "hears," clunim "I hear," clu "fame, glory," cluada "ears," Irish cloth "noble, brave;" Welsh clywaf "I hear," clod "praise, fame;" Old English hlud "loud," hlysnan "to listen, hear," hleoðor "tone, tune;" Old High German hlut "sound;" Gothic hiluþ "listening, attention."

"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 substrate 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. The 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). 

"knowing, wide-awake, untrusting, suspicious, alert," 1718, originally slang, with -y (2), but otherwise of unknown origin. Perhaps from dialectal lere "learning, knowledge" (see lore), or from leer (v.) in a now-obscure sense "walk stealthily with averted looks, sneak away" (1580s). OED suggests connection with archaic leer (adj.) "empty, useless," a general Germanic word (cognate with German leer, Dutch laar), of unknown origin.