'assistor' originates from Latin, specifically from the verb 'assistere' (via Old French 'assister'), where the elements 'ad-' meant 'to' and 'sistere' meant 'to stand'; the agentive suffix '-or' was added to form a noun meaning 'one who assists'.

'assistor' changed from Latin 'assistere' to Old French 'assister' and Middle English forms such as 'assisten/assisten' and eventually became the modern English noun 'assistor' by addition of the agentive suffix '-or'.

Initially it meant 'to stand near' or 'stand by', and over time the sense shifted to 'to help, give aid'; the noun form came to mean 'a person who helps' in modern usage.