'cosigner' originates from the prefix 'co-' meaning 'together' and the word 'signer', which comes from the Latin 'signare', meaning 'to mark or sign'.

'signare' transformed into the Old French word 'signer', and eventually became the modern English word 'signer'. The prefix 'co-' was added to form 'cosigner'.

Initially, it meant 'someone who signs together with another', and this meaning has largely remained the same in modern usage.