Felix said:
Illegetemis non corobundum.
---Don't let the bastards get you down
This is a sort of "comedy" pseudo-Latin, which has been mis-spelled, and accompanied with an incorrect pseudo-translation, to boot, thus ruining the joke altogether. Rather appropriate, considering.
The joke is thus:
Illegitimis non carborundum.
Don't let the bastards grind you down.
Unfortunately, it doesn't actually work in Latin. "Carborundum" is a trademark that was coined in the modern era for a specific formulation of silica carbide. And the Latin for "bastard" was either
nothus homo for a (male) bastard of a known father or
spurius for a (male) bastard of unknown father.
Illegitimus did mean "illegitimate", but more on the order of "not legal" or "not according to the forms".
The gist of the joke is that it is very much not Latin and very obviously not Latin to anybody who knows any Latin, but it might be thrown about by somebody who is trying to appear impressively educated and is so clueless as to not recognize such an obvious gaffe.