Refering to a person (more often than not a female), who is unusually gross and or disgusting.
Uh, dude look at that girl!
She's just a big bucket of yuck.
A symbol of, the worst of the worst of the yuckies. Like cleaning a hair ball out of the drain pipe in your shower after Seth shaves his pubes, or picking dried semen off the walls of his Suburu Baja, or possibly even using a man hole as a fleshlight. Yuck is life. Yuck is love. Big Yuck is the symbol of all thing yucky.
Oh wow, cleaning the long matted together Seth pube was the Big Yuck.