Derived from the word, curmudgeon, but pertaining to a person who is too young to act like they've got a "stick up their ***." Being too young to be stodgy or ultra-conservative (not politcally speaking). A person who acts older than their years. (i.e. people who, though not yet into "middle age" complain that every little ache and pain is likely to be terminal; also apt when discussing a person who won't attend an event/party on a weeknight, simply because they have to work the next day; also descriptive of a person who is unnecessarily argumentative in most conversations).
She's a kidmudgeon, when she refuses to go out on a weeknight, just because she works the next day.