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To take a piss with your trousers and underwear all the way down to your ankles. 'Big boy wees' are a humourous way to embrace the fact that some children don't know what the zipper is for, and do in fact wee like this.
Drunk guy: I'm going for a big boy wee on that tree!

Drunk guy 2: Me too, but maybe a different tree.
by Bagginski September 13, 2010
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