Side note: the word "mentalist" is widely-used London (maybe South-East London) slang for nutter/weirdo/unhinged person; the kind of person who is unpredictably or disproportionately angry, violent or prone to high-risk social behaviour that is best avoided. Usually a man: "bloke's a mentalist!"
It is basically never a compliment (whereas "mental" occasionally has a positive/fun connotation now).
It makes reading about Americans talking about "mentalists", including some of the comments on this page, very funny indeed.
To note that this is because people needed a word to describe someone who's "utterly mental" but didn't like "crazy" or "insane" (which are both considered not-PC these days, among other things), so they reached for the closest they'd heard - mentalist, made famous by the TV series - even though it was not the correct word.
But yeah, it is now in common usage. A bit like their/they're/there and "should of", it hurts my pedantic sensibilities, but in the end, people will speak how they want to speak and if enough of them agree, the language just changes.
No it's not, and I've used it long before that show was around. So much so that the title of that show was just as amusing as this title of this thread.
I especially enjoyed the trailers for tv show "The Mentalist" in this light.
I'm not as sure about the violence aspect of it, but certainly the rest rings true. Imagine the scene, you're driving in London and someone pulls out in front of you and does a series of weird manoeuvres, holding up the whole street for what feels like several hours. An appropriate exclamation could be "What the f*ck are you doing you utter mentalist?"
The Mentalist is such a guilty pleasure for me. The Sherlock/Watson combo is done to death in crime procedurals but as far as contemporary twists on the formula go, I found it a bit more memorable than, say, Elementary or Castle.
In my former South-West London circles, "mentalist" rarely had the negative connotation you describe. More frequently it was used to refer to someone's (contextually) outrageous/daft/extreme behaviour.
"You ran 15km before breakfast? You bloody mentalist"
"You carried on drinking even after I left? Mentalist"
It is basically never a compliment (whereas "mental" occasionally has a positive/fun connotation now).
It makes reading about Americans talking about "mentalists", including some of the comments on this page, very funny indeed.