The term "nutter" is a British slang used to describe someone who behaves in an eccentric, unconventional or unusual manner. It can also refer to someone who has lost touch with reality and is acting irrationally or erratically. The word is often used in a lighthearted way to describe someone's quirks or idiosyncrasies, but it can also be used in a more negative sense to imply that the person is mentally unstable or behaving strangely.