O.E. cene "bold brave," later "clever, wise," from P.Gmc. *kan- "be able to" (see
can). Original prehistoric senses seem to have been both "brave" and "skilled;" cf. O.N. kænn "skillful, wise," M.Du. coene "bold," O.H.G. kuon "pugnacious, strong," Ger. Kühn "bold,
daring." Sense of "eager" is from mid-14c. The meaning "sharp" is peculiar to Eng.: of blades and edges early 13c., of sounds c.1400, of eyesight c.1720. A popular word of approval in teenager and student slang from c.1900.
keen"lament," 1811, from Ir. caoinim "I weep, wail, lament," from O.Ir. coinim.