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A London way of saying 'na meen' which means you're being extra dumb and you know it. It's like when you're being a cocky arse and you think you're the best at everything.
'You think you're the best at this game? Nahtameen!'
'He said he'd win the bet, nahtameen? Typical idiot.'
'She kept talking about her cat like it was a god. Nahtameen!'