It means acting nuts.
To be a crazy person.
Matt: Oh, I can't believe you just jumped off that 80 feet building! You must be off your rocker!
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An idiot.
Usually used by british people.
Lea: I think I'm in love with Joe.
Sarah: but he's Kristens boyfriend! and she's your bestfriend!
Lea: Yeah, I know but who cares?
Sarah: oh, you are such a numpty.
To punch someone in the mouth to shut them up.
Tom: you're just jelous of me cuz I slept with your girl and you didn't. And y'know what? she liked it so much that I...
Edd: IF YOU DON'T SHUT UP, I'LL GIVE YOU A KNUCKLE SANDWICH
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