Okay, if “Illegitimi non carborundum” is fake Latin for “Don’t let the bastards grind you down” how would that statement actually be stated in real Latin?
Something like Noli permittere ut spurii te adterent would be close. “Do not (literally, “be unwilling”) to-allow that bastards you should-grind-down”. I just found “spurius” after a quick search, and now I think I know where one of the characters in “How Horatius Kept The Bridge” got his name. Grammatically, having said “be unwilling to permit” the thing you’re refusing to permit then appears as a subjunctive, not an infinitive and still less an indicative. Latin’s like that.
Hey, “istaimundus-fearriobundus!” Oh, I forgot, you don’t speak Latin… 