Old English toh "difficult to break or chew," from Proto-Germanic *tankhuz (cf. Middle Low German tege, Middle Dutch taey, Dutch taai, Old High German zach, German zäh). See rough for spelling change.
Figurative sense of "strenuous, difficult, hard to beat" is first recorded c.1200; that of "hard to do, trying, laborious" is from 1610s. Verb tough it "endure the experience" is first recorded 1830, American English. Tough guy first recorded 1932. Tough-minded first recorded 1907 in William James. Tough luck first recorded 1912; tough shit is from 1946.