Best definition
usually means: The union of gametes to form a zygote; conjugation.
In this case: (A type of HANGOVER) The one of

a kind, lingering,motor skill impairing,vegetating in bed all day, and hibernation causing HANGOVER you get from drinking Zygo Vodka.May also cause you to make strange complaining noises and grunts like:”whaaaa”or”ughhh”

“Holy hangover batman!..I have serious zygosis this morning..this open bar shit has to stop.”