1536, "born out of wedlock,” formed in Eng., modeled on L. illegitimus "not legitimate.” Sense of "unauthorized, unwarranted” is from 1645. Phrase illegitimati non carborundum, usually "translated" as "don’t let the bastards grind you down,” is fake Latin from c.1939. Carborundum was a brand of abrasives
(reg. trademark U.S. June 21, 1892, by Carborundum Co. of Monongahela City, Pa.). The compound was made from carbon and silicon; the word from carbon + corundum.