What is the origin of suffixing "-ass" to adjectives?

Solution 1:

It may have begun as part of a few stock phrases, but now I would say -ass functions as a generic intensifier. Colloquial English has a lot of these (one might say an assload), often making use of rude or vulgar words for added impact. One could as easily say fucking crazy, crazy as shit, crazy as hell, hella crazy, crazy as fuck, crazy as balls, or even combine them as balls-ass crazy.

Solution 2:

A little poking around on Google NGrams around gave 'stupid ass' as the oldest usage of the form I could find (1671). Which makes sense to me since it comes from comparing a person to the animal: Literally "a stupid ass."

The other forms are, I think, just derived from that original usage of 'ass' where other adjectives are substituted for the original 'stupid'. In effect, 'ass' becomes just a vulgar word for 'person'.