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1530s, "born out of wedlock," formed in English (and replacing earlier illegitime, c.1500), modeled on Latin illegitimus "not legitimate" (see il- + legitimate). Sense of "unauthorized, unwarranted" is from 1640s. Phrase illegitimi non carborundum, usually "translated" as "don't let the bastards grind you down," is fake Latin (by 1965, said to date from c.1939) (Carborundum was a brand of abrasives).