Urinating; also slang for being very annoyed or angry about something.
From Old French 'pissier,' of imitative origin (like 'piss-piss'), imitating the sound and action. Very old word dating back to at least 1300s.
One of the few English words that sounds like what it describes—linguists call this 'onomatopoeia'—and it's so ancient and imitative that nearly every European language has a nearly identical word (German 'pissen,' French 'pisser,' Dutch 'pissen').
Complete word intelligence in one call. Free tier — 50 lookups/day.