This is either the Ohio or the New Jersey version of whopperjawed. I had only ever heard it my way until I started looking it up to find the derivation. I mention Ohio because that's where I am from, but my mother who taught it to me is from New Jersey, with relatives from various east coast places.
A hardo is a person who tries extremely hard at everything. Most times a hardo will try very hard at things that do not require excessive effort. For example, sports practices, gym class, etc.
Wow did you see Jack at practice today? What a hardo!