Chapass is a condition that arises when: A: you have diarrhead so many times and wiped your anus raw, or B: when you forget to wipe your ass and walk around for the day. The end result is a sore and chafed ass that persists for 2 days at the least.
A chap ass is a word to describe someone being extremely annoying. In truth a chap is a type of legwear worn over jeans by cowboys, a chap ass is the same, but minus the jeans. So there ass in theory would be hanging out, therefor making it a chap ass.