the.com/bustier

means A fitted, often boned top that shapes the bust and torso and is worn as outerwear or a foundation garment, not tucked out of sight.

from French — simply the noun built from 'buste' (bust/chest, itself from Italian 'busto'), with the '-ier' suffix marking it as an object related to that noun, the same pattern you see in words like 'cartier' (card-maker) or 'bijoutier' (jeweler). So a bustier is, literally, 'the bust thing' — French fashion vocabulary doing what it does best, naming a garment plainly and letting the garment itself cause all the fuss.