Another word for nasty, but of a heightened nastiness. Old cheese is nasty. Old cheese with diarrhea on it is nersty. Often pronounced with a long "r" as in nerrrsty.
That movie was sick! But the part where his grandma vomited on the dog was nersty!
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).