Nothing. Identical in meaning to jack shit - it's just an equivalent for people who are scared of profanities.
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1. A homeless person.
2. British/Australian equivalent of "butt".
3. To fuck someone up the arse.
A bum bummed me up the bum.
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Posh for knob.
The above is an obscure reference to Harry Enfield or something.
"Penise?" "It's posh for knob."
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For every sexy action there is an equal and opposite sexy reaction.
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