eager beaver

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