A: Nobody uses the word 'ruly'.
B: I know, I'm just feeling gruntled.
A: Next thing you know, you'll be plussed.
I wouldn't call it a 'joke' really; just a (rather contrived and laboured - in my view) parading of three adjectives that are better known for their (negative) opposites - unruly, disgruntled, and non-plussed. The dialogue doesn't seem to me to have any meaning, so if I were you I wouldn't spend much time trying to understand it. Any online dictionary, though, would give you the meanings of those three words - and looking them up would be worthwhile.