How did urine become known as pee?

Answer by  Clement (1453)

"Pee" is an euphimism for "piss" registered already in 1788. "Piss" is derived from Old French (pissier,) originating from the Latin verb *pissiare that imitates the sound of peeing.

Answer by  jlmk (20)

It seems that "pee" is a short form of "piss". These shortenings come about when speakers want to refer to a concept without using the actual words, often because they are considered rude or unrefined. The first mention of "pee" as a word for urine is from 1902; the reduplicated "pee-pee" is from 1923.