: : What in (English-speaking) Europe is called the Great Northern Diver is called in North America the Common Loon (a species of bird). It has a weird, haunting cry which presumably suggested to someone the howls of the insane. Hence, crazy as a loon.

Why is someone called a loon?

The North American name "loon" likely comes from either the Old English word lumme, meaning lummox or awkward person, or the Scandinavian word lum meaning lame or clumsy. Either way, the name refers to the loon's poor ability to walk on land.

What does loon mean in slang?

(slang) A crazy or deranged person.