Usually defining a fair amount of something. The word is used with humorous intention as well as by people who are too coy to say shitload which is the more serious way to describe an amount.
adj. The greatest amount imaginable. Much more than a heap, a fuckton, an asston, an assload, and even a shitload, or any other word meaning "a great quantity."
A very large quantity in an unusual number or configuration (otherwise it would be an ordinary/English buttload). Compare: metric shitload and metric fuckton, which both designate a very large, but expected, quantity.