Green’s Dictionary of Slang

(orig. US) an excessively earnest, keen person whose efforts are sometimes more notable for their sound and fury than their actual usefulness.

1943		M. Hart Winged Victory I ii: They march off—six very, very eager beavers.
1950		Aus. Women’s Wkly 8 July 3/3: Don’t be an eager beaver.
1959		J. Thompson Getaway in Four Novels (1983) 74: Some eager beaver of a lab hound had managed to raise a latent print on the man’s corpse.
1960		J.P. Donleavy Fairy Tales of N.Y. I i: O.K. You don’t mind if we wait a few minutes. Until some of these eager beavers get off this pier.
1974		(con. 1945) E. Thompson Tattoo (1977) 270: A couple of eager beavers began to chip once more.
1985		B. Robinson Aussie Bull 7: In every job there are those who ‘wouldn’t work in an iron lung’, the ‘whingers’, the ‘practical jokers’, the ‘quiet achievers’, and the ‘eager beavers’.
1986		L. Heinemann Paco’s Story (1987) 22: The captain was antsy anyway, a real eager beaver.
1995		J. Stahl Permanent Midnight 284: Eager beaver that I was, I wanted to hop right behind that counter.
2005		B. Hughes Lighter Side of Firefighting 94: Eager Beaver A volunteer fire department had just bought a new ladder truck for their recently built fire hall. One of the eager guys on the squad jumped behind the wheel [etc].
2009		N. Flexner Disassembled Man [ebook] ‘I need to see her. Soon.’ ‘Well, well, well. You’re quite the eager beaver’.
2020		A. Parks Bobby March Will Live Forever 128: ‘Well, somebody’s an eager beaver.’ Jackson [...] took out a small glassine bag, held it up. Bobby grabbed it.
2022		A. Kirzman Giuliani 162: [H]e was out on the streets working as a ‘barker,’ beseeching pedestrians to come meet the candidate. The eager-beaver attitude paid off.
2023		M. Herron Secret Hours ‘I said, you bucking for a promotion, Sir Winston? [...] Because you’re certainly the eager beaver today’.