What it means
Nut is common British slang for your head. If someone says use your nut, they mean think straight, wise up, get your brain switched on before you do something stupid. As a verb, to nut someone means to headbutt them. Same word, totally different energy, so the sentence has to do the sorting.
Usage examples
"Use your nut and stop mouthing off to the bouncer, yeah? Too late, Gaz has nutted him outside the chippy and we’re legging it for the night bus."
"Use your nut, mate, you don't book a beach holiday in Scotland in November and expect a tan."
"He headed it straight in off his nut, best goal of the season and he barely even jumped."
"Oi, use your nut for once and check the train times before we trek all the way to the station."
"He got lippy in the queue and some idiot tried to nut him over basically nothing."
Where it comes from
Nut for "head" has been in English for centuries. It comes from the simple image of a head as a hard round thing, so the jump was pretty natural in everyday speech. From that noun sense, the verb nut later grew into the meaning "to headbutt" someone.
