What the adjective covers now
Beautiful is the ordinary English word for something that gives aesthetic pleasure: a face, a hillside, a melody, even a clean piece of reasoning. Dictionaries also keep a looser sense, generally pleasing or excellent, the kind of praise that can attach to a golf shot, a kitchen, or a stretch of weather. When the subject is a person, everyday speech still tends to use it more often of women than of men, though that is custom rather than grammar. Pretty, handsome, lovely, and fair remain nearby, each with a narrower job.
The spelling looks French because of the first syllable. The ending is not. The origin of the word beautiful is a hybrid built in English, not a ready-made adjective lifted from the Continent.
A native suffix on a borrowed noun
The adjective was formed inside English from the noun beauty plus the suffix -ful. The Oxford English Dictionary treats that as derivation, not borrowing, and its earliest evidence is around 1443, in the writing of Reginald Pecock, bishop of Chichester. Merriam-Webster likewise places first known use in the fifteenth century. Middle English spellings include bewteful and beautefull. By the middle of that century the sense already reached beyond the eye: pleasing to sight, hearing, mind, or soul.
The noun had come in earlier. Beauty is recorded from the early fourteenth century in forms such as bealte, beaute, and bewte, taken from Anglo-French beute and Old French biauté, earlier beltet. Modern French still has beauté. Those French nouns go back to a reconstructed Vulgar Latin abstract, bellitatem, the state of being pleasing to the senses. The same Latin noun yields Spanish beldad and Italian beltà.
English did what it often does once it owns a noun: it added -ful, full of. The suffix is native, from Old English -ful, itself from the adjective full. So the stem is Romance and the ending is Germanic. In this sense the new adjective largely pushed aside Old English fæger, the source of modern fair. The noun beauty, for its part, crowded out native wlite. English was not short of words for attractiveness; it imported a French prestige noun and then grew its own adjective from it.
Latin bellus, a small word for good
The French noun rests on Latin bellus, pretty, handsome, charming. In classical Latin that was not the high word for beauty. Pulcher and formosus carried more of that load. Bellus was lighter: used especially of women and children, and of men often with a sting of irony, closer to pretty than to noble-looking.
Most etymologists derive bellus from an Old Latin diminutive, duenelos, of duenos, good, the archaic ancestor of Latin bonus. The sound changes are regular for Italic: initial dw- becoming b-, and the cluster in the diminutive assimilating to a double l. The same family gives English, through Latin, bonus, benefit, and benevolent. If that account is right, the aesthetic sense is a later coloring of good or pleasing, not a separate invention.
The deeper Indo-European step is less secure. Some handbooks, following Watkins, link the Latin word to a root reconstructed as deu-, roughly to do, perform, show favor, or revere. Other treatments mark any Proto-Indo-European connection as doubtful. The Latin and Romance stages are firm; the Indo-European stage is a proposal, and it should be left that way.
Romance cousins of the adjective are easy to see: French beau and belle, Italian bello, Spanish bello, Portuguese belo. English later borrowed beau as a noun for a dandy or suitor. None of these is the same word as Latin bellum, war, which comes from Old Latin duellum. The two Latin words look alike in classical spelling. A minority view has even asked whether the war-word might be a euphemistic use of the same diminutive, something like quite brave recast as a name for fighting. The usual account keeps them apart. A folk link between beauty and war is not required and not safe.
How the English senses stretched
From the first English records, beautiful was not limited to looks. Mid-fifteenth-century use already allowed the ear, the mind, and the soul. Later centuries kept widening it. By the late sixteenth century it could praise moral or intellectual elegance, a proof as much as a painting. By 1756 it is also recorded as a noun, that which possesses beauty. Nineteenth-century English could place it after a noun for emphasis or irony, the house beautiful, a pattern John Bunyan had already used as a place-name, House Beautiful, in The Pilgrim's Progress. The fashionable set called the beautiful people appears in Vogue in 1964, after a 1941 play title by William Saroyan.
What did not happen is a collapse back into mere prettiness. Pretty stayed delicate, handsome stayed poised, fair kept a shade of freshness or fairness of hair and skin. Beautiful remained the broadest of the group: sensory, then mental, then a general intensifier of approval, and sometimes a raised eyebrow.
A leftover from Latin manners
One concrete remnant of the Latin usage still shows in English. Bellus was the social, slightly small word, not the solemn one. English beautiful grew into a large, serious adjective, but it never quite lost that coloring: it can praise a person, a theorem, or a day, and it can also be used with irony. The suffix -ful made it look home-grown. The stem still answers to French beau. Behind both sits a Latin diminutive of good.