Old English heard "solid and firm, not soft," also, "difficult to endure, carried on with great exertion," also, of persons, "severe, rigorous, harsh, cruel," from Proto-Germanic *hardu- (source also of Old Saxon hard, Old Frisian herd, Dutch hard, Old Norse harðr "hard," Old High German harto "extremely, very," German hart, Gothic hardus "hard"), from PIE *kortu-, suffixed form of root *kar- "hard."

Meaning "difficult to do" is from c. 1200. Of water, in reference to the presence of mineral salts, 1650s; of consonants, 1775. Hard of hearing preserves obsolete Middle English sense of "having difficulty in doing something." In the sense "strong, spiritous, fermented" from 1789 (as in hard cider, etc.), and this use probably is the origin of that in hard drugs (1955). Hard facts is from 1853; hard news in journalism is from 1918. Hard copy (as opposed to computer record) is from 1964; hard disk is from 1978; the computer hard drive is from 1983. Hard times "period of poverty" is from 1705. Hard money (1706) is specie, silver or gold coin, as opposed to paper. Hence 19c. U.S. hard (n.) "one who advocates the use of metallic money as the national currency" (1844). To play hard to get is from 1945. Hard rock as a pop music style recorded from 1967. To do something the hard way is from 1907.

Old English hearde "firmly, severely," from hard (adj.). Meaning "with effort or energy, with difficulty" is late 14c.

also hardheaded, 1580s, "stubborn," from hardhead "dull person" (1510s), from hard (adj.) + -headed. Meaning "practical, shrewd" is attested from 1779. Compare Dutch hardhoofdig "stupid."

also hardwired, 1969, in computing, "with permanently connected circuits performing unchangeable functions;" transferred to human brains from 1971; from hard (adv.) + wire (v.).

"tough, tough in a fight," literally "given to hard biting," 1715, originally of hunting dogs, from hard (adv.) + bitten, with the past participle used actively (as in free-spoken).

also hardworking, 1708, from hard (adv.) + working (adj.).

"uncompromising," 1958, originally in reference to Soviet communist policies, from the noun phrase (see hard (adj.) + line (n.)) in the political sense. Related: Hard-liner (1963).

of books, 1949, from hard (adj.) + cover (n.).

also diehard, 1844, in reference to the 57th Regiment of Foot in the British Army, from the verbal phrase die hard "suffer, struggle, or resist in dying," 1784; see die (v.) + hard (adv.). As an adjective, attested from 1871. The  brand of automobile battery, spelled DieHard, was introduced by Sears in 1967.

also hardboiled, 1723 in reference to eggs, "cooked so long as to be solid," from hard (adj.) + past tense of boil. In transferred sense "severe, tough," from 1886.