To urinate; a vulgar or informal way to describe going to the bathroom.
From Old French 'pissier' of uncertain ultimate origin. It appeared in English in the 13th century and was originally not considered vulgar, but gradually became informal/crude as the language evolved.
The word 'piss' started as a perfectly normal, non-rude word in medieval French and English, but over 700 years English speakers started treating it as crude slang—so the same word went from polite to insulting just because people's attitudes changed, not the word itself.
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