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.

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Definitions of keen from WordNet
keen (adj.)
very good;
Synonyms: bang-up / bully / corking / cracking / dandy / great / groovy / neat / nifty / not bad / peachy / slap-up / swell / smashing / old
keen (adj.)
having or demonstrating ability to recognize or draw fine distinctions;
Synonyms: acute / discriminating / incisive / knifelike / penetrating / penetrative / piercing / sharp
keen (adj.)
intense or sharp;
Synonyms: exquisite
keen (adj.)
painful as if caused by a sharp instrument;
keen winds
Synonyms: cutting / knifelike / piercing / stabbing / lancinate / lancinating
keen (adj.)
having a sharp cutting edge or point;
a keen blade
keen (v.)
express grief verbally;
Synonyms: lament
keen (n.)
a funeral lament sung with loud wailing;