The figurative bad guy who constantly dries your body out as you sleep after a night of heavy drinking, causing you to wake up every half hour or so feeling like you're stranded in the middle of the desert. You chug as much water (or any available liquid that's hopefully not more booze) as you can, and go back to sleep, and soon wake up gasping for more water and needing to urinate as well. This is because Dehydro is much better at drying you out than the water is at rehydrating you.
If you are so drunk that you manage to sleep through the night, Dehydro teams up with the toxins in your blood stream to give you a hangover worse than death.
Dehydro was a real bastard last night. I hardly got any sleep at all.