First used in Kurt Vonnegut's book, Breakfast of Champions, this phrase means roughly, "to take a leak."
"Sometimes somebody would say in his presence, 'Excuse me, I have to take a leak.' This was a way of saying that the speaker intended to drain liquid wastes from his body through a valve in his lower abdomen.
And Trout would reply waggishly, 'Where I come from, that means you're about to steal a mirror.'"
Guys, I'll be back in a second. I've gotta go steal a mirror.