'backrest' originates from English, specifically a compound of the words 'back' and 'rest', where 'back' comes from Old English 'bæc' meaning 'the rear part of the body' and 'rest' comes from Old English 'ræst'/'restan' meaning 'rest' or 'support'.

'back' in Old English was 'bæc' and 'rest' was 'ræst'/'restan'; these elements combined in later English to form the compound word 'backrest', which appears in post-medieval English as the modern 'backrest'.

Initially it simply meant 'a rest for the back', and over time the meaning has remained essentially the same, referring to the part of a seat that supports the back.