I think on the huh is east Anglian
I learned copacetic.from Canadian so yes birth American, and rather like the word.
Mumpsimus is from an apocryphal story told by Erasmus about an old priest whose prayer book had a misspelling, it should have been sumpsimus, but he said that he wasn't going to swap his old mumpsimus for the new sumpsimus
There is also a word sumpsimus which seems to have been also coined by Erasmus to mean the opposite, I found this definition
"adherence to or persistence in using a strictly correct term, holding to a precise practice, etc., as a rejection of an erroneous but more common form (opposed to mumpsimus) 2. a person who is obstinate or zealous about such strict correctness (opposed to mumpsimus)"
I'm sure that we all know someone who is a sumpsimus!