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."
The speed limit is 55, not 155, son. You're in a cubic buttload of trouble!
A very large quantity in an unusual number or configuration (otherwise it would be an ordinary/Englishbuttload). Compare: metric shitload and metric fuckton, which both designate a very large, but expected, quantity.