What does "merganser" mean?

Merganser: Any bird of the genus Merganser, and allied genera. They are allied to the ducks, but have a sharply serrated bill. Note: The red-breasted merganser (Merganser serrator) inhabits both hemispheres. It is called also sawbill, harle, and sheldrake. The American merganser (M. Americanus.) and the hooded merganser (Lophodytes cucullatus) are well-known species. -- White merganser, the smew or white nun.