To urinate; to pass liquid waste from the body.
Likely from the letter 'P' pronounced aloud ('pee'), which was used euphemistically in speech and later adopted as the actual word. It's a spelling pronunciation that became a proper word.
The word 'pee' is one of the few English words that literally came from saying a letter—kids probably started saying 'P' as a silly way to talk about bathrooms, and it stuck around until adults started using it too.
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