Eager
Middle English (also in the sense ‘pungent, sour’): from Old French aigre ‘keen’, from Latin acer, acr- ‘sharp, pungent’.
wiktionary
From Middle English egre, eger, from Old French egre (French aigre), from Latin acer(“sharp, keen”); see acid, acerb, etc. Compare vinegar, alegar.
See eagre.
etymonline
eager (adj.)
late 13c., "strenuous, ardent, fierce, angry," from Old French aigre "sour, acid; harsh, bitter, rough; eager greedy; lively, active, forceful," from Vulgar Latin *acrus (source also of Italian agro, Spanish agrio), from Latin acer "keen, sharp, pointed, piercing; acute, ardent, zealous" (from PIE root *ak- "be sharp, rise (out) to a point, pierce").
Meaning "full of keen desire" (early 14c.) seems to be peculiar to English. The English word kept a secondary meaning of "pungent, sharp-edged" till 19c. (as in Shakespeare's "The bitter clamour of two eager tongues," in "Richard II"). Related: Eagerly; eagerness. Eager beaver "glutton for work" [OED] is from 1943, U.S. armed forces slang.
