'reassurer' originates from modern English, specifically from the verb 'reassure', where the prefix 're-' meant 'again' and 'assure' meant 'to make secure or confident'.

'assure' came into English via Old French 'asseurer' (from Latin roots related to 'securus'/'securare'), and 'reassure' was formed by adding the prefix 're-' to that verb; 'reassurer' is the agent noun formed from 'reassure'.

Initially, the root meant 'to make secure' (to put into a state of safety); over time it evolved into the current sense of 'restore confidence or calm someone's fears'.