eager beaver

a (real) eager beaver (a person who wants to succeed) — трудяга; излишне усердный, ретивый работник; человек, выслуживающийся перед начальством

Example: He gets to work every day before eight. He's a real eager beaver.

Well, as I see it 'hard worker' differs from 'eager beaver' slightly. First of all, an eager beaver wants to succeed too much while a hard worker just works hard (no nuances). 'Eager beaver' is often used a bit derogatorily — a person who is willing to work very hard and always ready to please other people ...

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from "beaver away" — вкалывать)
Related vocabulary
[glutton for punishment]