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.
Drunk guy 2: Me too, but maybe a different tree.
by Bagginski September 13, 2010
Get the Big Boy Weemug.