'aplomb' originates from French, specifically the word 'aplomb', where 'a-' (from à) meant 'to/toward' and 'plomb' meant 'lead' (the weight used on a plumb line), ultimately from Latin 'plumbum' meaning 'lead'.

'aplomb' was used in French to mean 'perpendicularity' or 'the state of being plumb' (i.e., determined by a plumb line); it was borrowed into English in the early 18th century with senses of steadiness or vertical alignment and later came to be used figuratively for composure and assurance.

Initially it meant 'perpendicularity' or 'the quality of being plumb (steady/vertical)', but over time it evolved into the modern sense of 'self-confidence' or 'composure'.