'backless' originates from Old English components: 'back' + the suffix '-less', where 'back' came from Old English 'bæc' meaning 'the back (of the body or an object)' and the suffix '-less' came from Old English '-lēas' meaning 'without'.

'back' developed from Proto-Germanic '*bakam' into Old English 'bæc', while the suffix '-less' comes from Old English '-lēas' (from Proto-Germanic roots meaning 'free from'); these elements combined in Middle English to form the compound 'backless' and continued into modern English.

Initially formed simply as a literal compound meaning 'without a back', and over time it has retained that core meaning while also being applied specifically to garments (meaning 'with the back exposed').