Keen
Old English cēne ‘wise, clever’, also ‘brave, daring’, of Germanic origin; related to Dutch koen and German kühn ‘bold, brave’. Current senses date from Middle English.
wiktionary
From Middle English kene(“ bold, brave, sharp”), from Old English cēne(“keen, fierce, bold, brave, warlike, powerful; learned, clever, wise”), from Proto-Germanic *kōniz(“ knowledgeable, skilful, experienced, clever, capable”), from Proto-Indo-European *ǵneh₃-(“to know”). Cognate with Danish køn(“ handsome, pretty”), Dutch kien(“ smart, wise, able”), koen(“ daring, valiant, doughty, courageous”), German kühn(“bold, daring, audacious, hardy, valiant, venturesome”), Icelandic kænn(“wise, crafty, clever, able”), Scots keen(“ lively, brisk; avaricious”). Related to Old English cunnan(“to know how to, be able to”). More at cunning, can.
From Irish caoin(“to cry, weep; to keen”).
etymonline
keen (adj.)
c. 1200, kene, from Old English cene "bold, brave, fearless," in later Old English "clever, prudent, wise, intelligent," common Germanic (cognate with Old Norse kænn "skillful, wise," Middle Dutch coene "bold," Dutch koen, Old High German kuon "pugnacious, strong," German kühn "bold, daring"), but according to OED there are no cognates outside Germanic and the original meaning is "somewhat obscure"; it seem to have been both "brave" and "skilled." Perhaps the connection notion was "to be able" and the word is connected to the source of can (v.1).
Sense of "eager (to do something), vehement, ardent" is from c. 1300. The physical meaning "sharp, sharp-pointed, sharp-edged" (c. 1200) is peculiar to English. Extended senses from c. 1300: Of sounds, "loud, shrill;" of cold, fire, wind, etc. "biting, bitter, cutting." Of eyesight c. 1720. A popular word of approval in teenager and student slang from c. 1900. Keener was 19c. U.S. Western slang for a person considered sharp or shrewd in bargaining.
keen (v.)
"lament loudly over the dead, bitterly wail," 1811, from Irish caoinim "I weep, wail, lament," from Old Irish coinim "I wail." Hence "to utter in a shrill voice" (1893). Related: Keened; keener; keening. As a noun from 1830.
