eageryoudaoicibaDictYouDict[eager 词源字典]
eager: [13] As its close etymological connection with vinegar and acid might suggest, the underlying sense of eager is ‘sharp’. It comes ultimately from the Indo-European base *ak- ‘sharp, pointed’, amongst whose other English descendants are acne, edge, and oxygen. It was the source of Latin ācer ‘keen, sharp’, which was used in relation both to sight, hearing, etc, and to temperamental qualities – hence ‘ardent, zealous’.

The Latin adjective (from which English also gets acid and acrid) became *acrum in post-classical times, and from this came Old French aigre (source of the -egar of vinegar), which passed into English via Anglo- Norman egre. English retained the literal senses ‘pungent, sour’ and ‘sharp-edged’ until the early 19th century.
=> acid, acne, acrid, acute, edge, oxygen[eager etymology, eager origin, 英语词源]
eager (adj.)youdaoicibaDictYouDict
late 13c., "strenuous, ardent, fierce, angry," from Old French aigre "sour, acid; harsh, bitter, rough; eager greedy; lively, active, forceful," from Latin acrem (nominative acer) "keen, sharp, pointed, piercing; acute, ardent, zealous" (see acrid).

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.