"Restaurant" entered English in 1806, borrowed straight from French — and at first, it was very much felt as a foreign word, used specifically to describe French-style eating establishments. The French "restaurant" originally didn't mean a place at all — it meant "food that restores," describing a bouillon or broth meant to restore your strength. That word came from "restaurer," "to restore or refresh," which traces back further to Old French "restorer" (the same root that gives us the English word "restore").
The shift from "restorative broth" to "restaurant as a place" happened in Paris around 1765. According to one 18th-century account, a man named Boulanger set up shop selling these restorative broths and consommés, and posted a cheeky Latin sign paraphrasing scripture: "Come to me, all you who suffer from stomach ailments, and I will restore you" — playing on the word "restaurabo." His success spawned imitators calling themselves "restaurateurs," and the name stuck to the type of establishment rather than the soup itself. (Other historical sources credit different Paris restaurant pioneers, but Boulanger became the standard legend.) Because English adopted the word wholesale from French, it kept its French spelling — including that silent-ish nasal "-ant" ending — rather than being reshaped to look Latin or Greek. By the 20th century, English also picked up the Italian version, "ristorante," and by the 1870s, the word had extended to "restaurant car" on trains. Interestingly, English once had its own native cousin, "restaurance," meaning "restitution" in legal contexts, but that word died out and left the French import to take over the dining sense entirely.
restaurant (n.) "an eating-house, establishment where meals may be bought and eaten," by 1806, in a French context, from French restaurant "a restaurant," originally "food that restores," noun use of present participle of restaurer "to restore or refresh," from Old French restorer (see restore ). Les restaurans ont donné naissance aux Restaurateurs. C’est un établissement qui a eu lieu à Paris vers 1765, & qui fut imaginé par un nommé Boulanger, lequel demeurait rue des poulies. Sur sa porte, il avait mis cette devise, qui était une application peu respectueuse d’un livre très-respectable: Venite ad me omnes qui stomacho laboratis, & ego restaurabo vos. Boulanger vendait des bouillons ou consommés. On trouvait même chez lui à manger quand on voulait. Il est vrai que n’étant point Traiteur, il ne pouvait servir de ragoûts; mais il donnait des volailles au gros sel, avec des œufs frais; & tout cela était servi proprement sur ces petites tables de marbre, connues dans les caffés. A son imitation, s’établirent bientôt d’autres Restaurateurs. [...] La nouveauté, la mode, & peut-être même leur cherté, les accréditèrent: car ce qu’ils fournissaient était plus cher que chez les Traiteurs ordinaires. Mais telle personne qui n’eût point osé aller s’asseoir à une table d’hôte pour y dîner, allait sans honte dîner chez un Restaurateur. [M. Le Grand d'Aussey, Histoire de la Vie Priveés de Français, Tome II. 1782.] [ Restaurants gave birth to Restaurateurs . This is an institution that began in Paris around 1765, and was the brainchild of a man named Boulanger, who lived on Rue des Poulies. On his door, he had put this motto, which was a disrespectful application of a very respectable book: Venite ad me omnes qui stomacho laboratis, & ego restaurabo vos. Boulanger sold broths or consommés. One could even find something to eat when one wanted. It's true that as he wasn't a Traiteur , he couldn't serve ragoûts ; but he did serve poultry in coarse salt, with fresh eggs; and all this was served neatly on those little marble tables known from the cafés. Other Restaurateurs soon followed in his footsteps. [...] Novelty, fashion, and perhaps even their high prices, accredited them: for what they provided was more expensive than that of ordinary Traiteurs . But a person who would not have dared to sit down to dinner at a table d'hôte , would go without shame to dine at a Restaurateur's .] The identification of Boulanger was unusual before 1780s; earlier accounts from 1760s-1770s variously name Duchêne, Roze or Vacossin as the premier restaurateur from Rue des Poulies. For the restaurateur's rival the traiteur , see trattoria . Italian spelling ristorante attested in English by 1925. Middle English had similar words in legal language, such as restaurance "restitution." The railroad restaurant car (1872) was one adapted to afford meals to passengers while travelling. also from 1806