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.'"