Whatever bodily liquid you wake up in from the night before.

when you're hungover as shit after a spending a night drunk as a skunk

that glorious first piss of the morning.

NOTICE: This word has an abbreviation. It is Mater pronounced Mah-ter, it rhymes with water. Now for the definition.
The shit you drink when you first wake up because your throat is dry as a bone. Morning water tastes like uh-oh stinky 97% of the time. If it tastes good, consider yourself very lucky. Night water is better.