Take a pee

Does anyone know where the phrase "take a pee (piss) comes from?

You've answered your own question, now haven't you? "Piss" is considered vulgar and harsh to say, and (in many circles) abbreviating the word to "P" of "Pee" is 73.56% less vulgar. Roughly. Now, if you'll excuse me, I have to ... oh, tap a kidney.