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]

If you describe someone as an eager beaver, you mean that they are very enthusiastic about work and want very much to please other people. There are always eager beavers, people who stay behind after the talk to ask penetrating questions. Ed was the first to arrive at the office, the eager beaver! Note: Eager-beaver can also be used before a noun. If fraud became an issue, he might interest an eager-beaver lawyer in the case. Note: You usually use these expressions to show that you find someone's behaviour foolish or annoying. Note: Beavers are often associated with hard work, as they spend a lot of time building shelters and dams (= walls across rivers) out of mud and wood.

An overzealous or extremely ambitious individual. The beaver has been known as an especially hard worker since at least the seventeenth century, on a par with busy as a bee. It was only in the twentieth century that this not-quite-rhyming expression gained currency. It was widely used for overzealous recruits during World War II who chronically disobeyed the unwritten rule, Never volunteer, and rapidly became a cliché.