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1.
The process of drinking large amounts of water or other hydrating fluids (gatorade or powerade) prior a period of heaving drinking, as an attempt to lessen the severity of a hangover, despite getting bombed, trashed or shit faced.

The unfortunate side effect of prehydrating is numerous trips to the bathroom once you break the seal.
We are starting the party early tonight, you need to prehydrate so you don't wake up with a killer hangover tomorrow.
by LTC3 July 21, 2008
 
2.
When you drink a lot of water or other liquids in preparation for a night of heavy drinking in the hopes that you won't wake up with a desert in your mouth the next day.

aka 'prydrate'
Rodney: "Hey Herbert, you sure are drinking a lot of tea"

Herbert: "Yeah, I'm going to that Japanese whiskey bar Ichikura tonight, so I wanna pre-hydrate"