What English uses it for now
In everyday English, hard names whatever resists a push: stone, ice, a locked door. The same word also names what resists the mind or the will: a hard problem, a hard winter, a hard person to reach. It can describe water loaded with mineral salts, a consonant with a stop quality, news that is factual rather than decorative, or drink that has been fermented or distilled. As a bare adverb it still means with force or effort: hit hard, work hard, look hard.
Those uses look scattered. They are not separate loans stitched together later. They grew from one native adjective that English has carried since the Old English period.
Native, not borrowed
The origin of the word hard is inheritance, not a medieval import. Old English had heard for matter that was solid and not soft, for experience that was severe or painful to bear, and for people who were stern or cruel. The matching adverb was hearde, already meaning firmly or severely. The Oxford English Dictionary places both adjective and adverb before 1150 and treats the word as Germanic by descent.
The spelling we use now is a later smoothing. In Old English the vowel of heard was a broken diphthong before r plus a consonant. Middle English reduced that sequence, and the form settled as hard (sometimes harde). The comparative and superlative stayed on the native pattern, giving modern harder and hardest rather than a Romance-style more/most pair as the default.
Nothing in that path is a French or Latin loan into the core adjective. English did later take in a close relative through French, but that is a second arrival, not the source of the everyday word.
The Germanic stem
Comparative reconstruction sets Old English heard beside a large West and North Germanic family. Dutch still has hard, German hart, West Frisian hurd, and the Scandinavian languages have forms such as Icelandic harður and Swedish and Danish hård. Gothic, the earliest well-attested East Germanic language, has hardus. Those matches point back to Proto-Germanic *harduz, a u-stem adjective reconstructed with the senses hard and, in some descendants, brave.
From that stem English also built verbs of becoming and making: Old English heardian and hierdan, the ancestors of harden. Related names and compounds in the Germanic languages used the same element for toughness of character as well as of material. The picture is consistent: a common North-Sea and inland Germanic word for what does not give way.
An older root that is not settled
Beyond Germanic the trail is less firm. Many handbook etymologies, including the American Heritage treatment of the root kar- or ker- and the account on Etymonline, take *harduz from a suffixed Indo-European form on the order of *kortu- and link it with Greek kratos (strength, power) and kratys (strong). On that view the Germanic adjective and the Greek power-words share a prehistoric idea of hardness or might.
Specialists do not all accept that equation. The Wiktionary reconstruction of *harduz records the traditional comparison with Greek, then notes that Guus Kroonen, following earlier work by Strunk and Heidermanns, treats a derivation from the cutting root *(s)ker- as a live alternative, with possible Baltic and Slavic comparisons such as Lithuanian kartùs (bitter). Other scholarship has also questioned whether Greek kratys and Gothic hardus are the same word historically, because the vowel grades and the oldest Greek meanings do not line up neatly.
The honest statement is therefore modest. Germanic *harduz is secure. A deeper Indo-European parent is proposed, and Greek strength-words are the usual candidates, but that link is a scholarly hypothesis, not a closed fact. Where the evidence thins out, it is better to stop than to invent a single tidy tree.
How the senses spread
Old English already held more than the tactile meaning. Heard could describe a harsh master or a bitter ordeal as readily as a hard surface. Around 1200 a further step is recorded: hard as difficult to accomplish, not only difficult to suffer. That is the main modern intellectual sense, the one in a hard exam or a hard climb.
A Middle English use meaning having trouble doing something survives almost only in the set phrase hard of hearing. The rest of that construction faded, which is why the phrase now sounds slightly fossilized even to speakers who use it every day.
Later English kept attaching the adjective to new domains without changing its core image of resistance. From the 1650s, hard water is water that will not lather easily because of dissolved salts. From 1775, a hard consonant is one felt as firm or stopped rather than hushed. From 1789, hard of cider and later of liquor marks a drink that is spirituous rather than merely sweet; that line is generally taken as the model for hard drugs in the mid-twentieth century. Hard times for a stretch of want is attested from 1705; hard money for coin as against paper from 1706.
Technical and journalistic English added more overlays: hard facts in the nineteenth century, hard news in the early twentieth, then hard copy, hard disk, and hard drive as computing needed a word for storage you could treat as physical. None of these is a new etymology. Each is the old adjective applied to a new kind of stubbornness: of evidence, of media, of metal.
The adverb hard kept the old force of hearde: with energy, with impact. The derived adverb hardly did not. Old English heardlice meant sternly, severely, or with great effort. By the early fifteenth century it could mean not easily, and by the 1540s the dominant sense had flipped to barely or scarcely. That is why work hard and hardly work are not polite variants of each other. One kept the ancestral force; the other passed through difficulty into near-negation.
The French cousin
English did borrow a sibling. Around 1200 hardy entered from Old French hardi, the past participle of hardir (to harden or make bold), itself from a Frankish or other Germanic verb built on the same *harduz stem. So hardy is the native idea sent out into Romance and returned as a word for boldness, and later for plants that can stand a winter outdoors.
That split is a useful last fact. Hard is the home form, still naming what will not yield. Hardy is the same Germanic toughness after a French detour, now specialized for courage and endurance. Speakers who feel them as near-synonyms are not wrong about the history, only about the route.