eager beaver

One who is industrious and enthusiastic (perhaps annoyingly so). Give that big project to the new girl, she's a real eager beaver. Aren't you an eager beaver, getting to the office an hour early.

someone who is very enthusiastic; someone who works very hard. New volunteers are always eager beavers. The young assistant gets to work very early. She's a real eager beaver.

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]