hardyoudaoicibaDictYouDict[hard 词源字典]
hard: [OE] Hard comes ultimately from a prehistoric Indo-European *krátus, which denoted ‘power, strength’. This original meaning was carried over into Greek krátos ‘strength, power, authority’ (source of the ending -cracy in such English words as democracy and plutocracy), but the Germanic languages took it over mainly in the sense ‘resistant to physical pressure’.

The prehistoric Germanic form *kharthuz produced, besides English hard, German hart, Dutch hard, Swedish hård, and Danish haard. The sense ‘difficult’, incidentally, developed in the 14th and 15th century from the notion ‘resistant to one’s efforts’. A Germanic derived verb *kharthjan ‘harden’ was borrowed into Old French as hardir ‘embolden’, and its past participle hardi ‘bold’ reached English as hardy [13].

Its main modern sense, ‘robust, tough’, presumably a harking back to its distant English relative hard, developed in the 16th century.
=> hardy[hard etymology, hard origin, 英语词源]
hard (adj.)youdaoicibaDictYouDict
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- (cognates: 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- (cognates: Greek kratos "strength," kratys "strong"), suffixed form of root *kar-/*ker- "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, 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.
hard (adv.)youdaoicibaDictYouDict
Old English hearde "firmly, severely," from hard (adj.). Meaning "with effort or energy, with difficulty" is late 14c.