eager beaver

n. Informal
One that is exceptionally, often excessively industrious or zealous: "The eager beavers of industry seldom reach their potential, much less rise to the top" (Newsweek).

eager-beaver ea'ger-bea'ver

Very hard-working individual, anxious to succeed. The person puts in many hours and is always busy. There is a strong desire for promotion and high compensation.

noun

An intensely energetic, enthusiastic person: dynamo, hustler. Informal go-getter, live wire. See concern/unconcern.

An exceptionally zealous person, one who habitually takes on more tasks or works harder than others. For example, Bill is a real eager beaver, always volunteering to stay late. This expression became especially popular during World War II, applied to recruits anxious to impress their commanding officers by such behavior. [First half of 1900s]

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The noun has one meaning:

Meaning #1: an alert and energetic person
  Synonyms: busy bee, live wire, sharpie, sharpy

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