adding ass to a word increases its intensity, much as a superlative or other such quantifier would. if something is funny-ass shit it is more funny than if were described as merely funny shit. the same goes for stupid ass shit or stuff. ass is usually a compound suffix with another noun word like shit, stuff, drink, etc. as in, that was a delicious ass soda. do not try explaining this particular use of this word to people who speak another language unless you want hilarity to ensue. there are rules governing its useage, but they are unclear.
that was a weak ass attempt at a definition.