I have done some research, but I cannot find the origin of this phrase. It is very common usage in America, especially among children. Some people recount being taught in Kindergarten to hold up one finger if you had to go to pee (urinate) or two fingers if you had to poo (defecate). Going number one always means urination number two always means defecation. I don't really have any way to help you remember which is which. Ask any American child and they can tell you. LOL. Troy, you are very incorrect and way off base! You could not be more wrong. This gesture was originated in kindergarten classes of small children!

number two = poo This expression was probably taken over on the Mayflower.

#1=Pee The middle finger, a single, one finger gesture, looks like the male penis (also means F#$% You [sexual innuendo...implied], an offensive American gesture) #2=Poo The only bathroom alternative to #1.