Nutcase is a slang name for somebody behaving wildly. An absolute nutcase, some nutcase in a van, calling him a nutcase.
It is one of a family of words — headcase, mental case — that treat mental illness as an insult, which is why campaigners have pushed back on all of them. Used about a friend doing something reckless it is affectionate; used about a stranger it is not.
1nounnutcaseA person whose behaviour is wildly eccentric or unhinged.
“Some nutcase was driving the wrong way down the motorway.”
nutcase is an insult.
Nut plus case, twentieth-century slang. Nut for the head goes back to the nineteenth century.
From nut + case. Compare headcase, mental case etc.
Etymology from Wiktionary, CC BY-SA