Origins
The word 'beautiful' is a hybrid formation, combining a French-derived base with a native English suffix. 'Beauty' entered English from Anglo-Norman 'beauté' (Old French 'bealté,' later 'beauté'), which descended from Vulgar Latin *bellitātem, an abstract noun derived from Latin 'bellus,' meaning 'pretty,' 'handsome,' or 'charming.' The English suffix '-ful' (from Old English '-full,' meaning 'full of') was then attached to create 'beautiful' — literally 'full of beauty.'
Latin 'bellus' was originally a diminutive of an older form 'duenelos,' related to 'bonus' (good). In classical Latin, 'bellus' was somewhat informal — the standard literary word for 'beautiful' was 'pulcher' (feminine 'pulchra'), which gave English 'pulchritude.' But in the spoken Vulgar Latin of the late Roman Empire, 'bellus' overtook 'pulcher' in everyday use, and it is 'bellus' that survived into the Romance languages: French 'beau/belle,' Italian 'bello/bella,' Spanish 'bello/bella,' Portuguese 'belo/bela,' Romanian 'bel/belă.'
The word 'beautiful' first appeared in Middle English around 1440. It was not the first English word for aesthetic excellence — 'fair' (from Old English 'fæger') had served that role for centuries and remains in use ('fair maiden,' 'fairest of them all'). 'Comely,' 'handsome,' 'lovely,' and 'pretty' were also available. But 'beautiful' gradually rose to become the primary, most versatile term for supreme aesthetic quality, perhaps because its three syllables gave it a weight and emphasis that shorter words lacked.
Latin Roots
The suffix '-ful' is a distinctly Germanic element. If 'beautiful' had been formed entirely from French or Latin components, it might have taken a form like 'beauteous' — which does exist in English, appearing around the same time, but sounds more literary and archaic. The parallel form 'beautific' was never adopted (though 'beatific,' from a related Latin root 'beātus,' meaning 'blessed,' exists). The success of 'beautiful' over its competitors is a case study in how English freely mixes its Germanic and Romance building blocks.
The word 'embellish' (from Old French 'embellir,' to make beautiful, from 'en-' + 'bel/belle') shares the same Latin root. 'Beau' (a suitor, a dandy) was borrowed directly from French in the seventeenth century. 'Belle' (a beautiful woman) followed. 'Belle époque' (beautiful era), 'beau monde' (beautiful world, i.e., fashionable society), and 'belles-lettres' (beautiful letters, i.e., fine literature) are French phrases that entered English wholesale.
Philosophers have debated the nature of beauty — and whether 'beautiful' describes an objective quality or a subjective response — since Plato. The word itself takes no sides in this debate. It simply asserts that something is full of beauty, leaving the listener to decide what beauty is.