Used in British English to refer to someone who is insane.
Also probably the most common mistake made by outsiders when trying to reproduce a British accent. For the record, it's either "he's a nutter" or "he's nuts". Saying "he's nutters" is a good way to make British people dislike you.
Good: "What are you, some kind of nutter?"
Bad: "Are you completely nutters?"
An absolute loose cunt who does the sickest and funniest shit and is always good to have around for a laugh. Everything a Nutter does is shaded with a tinge of crazy. Lacks awareness and is a lunatic, doesn't have it all quite there upstairs, but will act like a muppet, show off and be the nuttiest nutter.
Sammy: Did you see my Dragon Knight last night? Absolute nutter, mate. Sschliiiiike!