What speakers mean by it now
In present-day English, terrible is first of all a word for something very bad. A terrible film, a terrible idea, or a terrible headache can be unpleasant, incompetent, or simply intense. The older, more literal force is still available: a storm, a wound, or a crime can be terrible because it frightens. More often the adjective works as a general intensive, marking degree rather than panic. It can describe a cost paid, a strain endured, or a person who is simply no good at a task. That spread of senses is not a modern slang accident. It is what happens when a word that once named dread is used, for centuries, as a way of saying how much.
French, Latin, and a late medieval arrival
The origin of the word terrible is a double borrowing rather than a single leap. English first records the adjective around 1400, in the Middle English period. Historical dictionaries treat it as coming from two directions at once: Anglo-Norman and Middle French terrible, and classical Latin terribilis. Old French already had the form in the twelfth century, with an early date around 1160, in the sense of causing terror or being fit to cause it. Later medieval French also used it of what was awe-inspiring, then of what was harshly severe or painful.
Latin terribilis meant inspiring terror or alarm. English therefore did not wait for a purely scholarly import. Clerks and translators could take the Latin adjective directly, while ordinary written English also received the French shape that had already been current for two centuries. One modern etymological dictionary notes that the incoming Romance word largely displaced native Old English adjectives such as atol and sliþe and took over much of the work once done by grimm. English did not lack words for what was grim or hideous; it replaced a stretch of that older stock with a French-Latin form that sounded like the rest of the new Romance layer in the language.
The verb, the suffix, and a root of trembling
The Latin adjective is built in a way that is still readable. Terribilis rests on the verb terrēre, to frighten or fill with fear, plus the suffix -bilis, the same ending that yields English -ble and -ible in horrible, possible, and many others. The combination is roughly fit to frighten, capable of producing dread. A surface analysis of the English word as terror plus -ible is close enough to be useful, even though the historical formation sits on the verb rather than on the noun.
That verb belongs to a tight Latin family. Terror is the state of intense fear. Later English relatives include terrify, terrific, and deter, the last from dēterrēre, to frighten someone off. Romance siblings of the adjective are easy to see: French terrible, Spanish terrible, Italian terribile, Portuguese terrível. They are not later copies of the English word; they are parallel descendants of the same Latin adjective.
Behind Latin, historical linguists reconstruct an Indo-European root connected with trembling and fear. The exact formula is not settled. Some accounts write *tres- for quivering or trembling; others cite *tros- in the sense of making afraid; older comparative handbooks list related shapes. The reconstruction should be treated as a working hypothesis, not as a recorded word. The comparative evidence is firmer. Sanskrit has a verb for trembling and being afraid. Avestan has a form meaning scared. Greek has a verb for trembling in fear. Lithuanian has a verb for shivering. Old Church Slavonic has a first-person form meaning I shake. Middle Irish has an adjective for timid. English terrible sits at the far end of that chain, after Latin had already turned the idea of shaking with fear into the idea of causing it.
How the sense spent down its terror
The earliest English uses stay close to the Latin: causing terror, fitted to excite awe or dread, frightful, even unendurable. That is a serious adjective, usable of judgment, war, weather, or a face that cannot be looked at steadily. By the 1590s it had stretched to sources of feelings akin to dread, including violent severity. In the eighteenth century the force weakened further into a mere intensive: great, severe, excessive. A terrible bore need not terrify anyone. The word is there to mark degree. Awful and dreadful walked a similar path, losing much of their original weight.
A related Latin adjective, terrificus, meaning causing terror, produced English terrific, which later took a sharper turn. By the early nineteenth century it could mean very great; by the late nineteenth it could mean excellent. English terrible did not make that last flip. It stayed on the negative side of the intensive. By the early twentieth century, in colloquial use recorded around 1913, it was an ordinary word for very bad or extremely incompetent. The phrase terrible twos, for a difficult stage of toddlerhood, is attested by 1949. French, by contrast, still allows colloquial terrible as praise: a concert can be terrible and that can mean it was outstanding. The same Latin seedling grew opposite evaluations. English kept the pejorative and farmed the compliment out to terrific.
What the native words left behind
A useful leftover of the history is not a date but a substitution. Before the French and Latin form settled, English already had native adjectives for what was grim, cruel, or hideous. The borrowed word did not invent dread. It replaced much of that older vocabulary and then, over six centuries, spent down its terror until it could describe a meal, a performance, or a two-year-old. The morphology still says fit to frighten. Everyday use often says only that something failed.