But it has a definite meaning historically since at least WWII.  And at Harvard.  

https://en.wikipedia.org/wiki/Illegitimi_non_carborundum

There are many variants of the phrase, such as
Non illegitimae carborundum
Noli illegitimi carborundum
Nil illegitimi carborundum
Non illegitimis carborundum
Illegitimi nil carborundum
Nil bastardo carborundum
Nolite te bastardes carborundorum
Nil carborundum ab illegitimati
Illegitimis non carborundum
Nil illegitimo in desperandum carborundum
Nil carborundum illegitamae
Nec Illegitimi carborundum
Nolite Illegitimos Conterere Vos
Non carborundum bastardum

None of these variants is 'legitimate' Latin any more than the original