'chaplain' originates from Old French, specifically the word 'chapelain', where the Latin root 'capella' (via Medieval Latin 'cappella') meant 'little cloak' or 'chapel'.

'chaplain' changed from the Medieval Latin word 'cappellanus' (meaning a keeper of a 'cappella' or chapel/cloak) into Old French 'chapelain', passed into Middle English as 'chapelain', and eventually became the modern English word 'chaplain'.

Initially it referred to the guardian or custodian of a chapel (and by association the 'little cloak' of St. Martin kept as a relic); over time the meaning shifted to denote a clergy member attached to an institution who conducts religious duties and pastoral care.