'bailor' originates from Old French, specifically the word 'baillier' (also attested as 'bailer'/'baillier'), where the root 'baill-' meant 'to control, to deliver or have charge of'.

'bailor' changed from Middle English 'bailour' (or 'baylour'), which in turn came from Old French forms, and eventually became the modern English word 'bailor'.

Initially, it meant 'one who has charge of or delivers goods', and over time the meaning narrowed to the legal sense 'the person who entrusts goods to another (the bailee)', which is the current usage.