'assistor' originates from Late Latin/Old French influence, specifically from the Latin verb 'assistere' (via Old French 'assister'), where the prefix 'ad-' (appearing as 'a-') meant 'toward' and 'sistere' meant 'to stand'.

'assistere' passed into Old French as 'assister' and into Middle English as forms like 'assisteren' or 'assister', with the English agentive suffix '-or' producing 'assistor' in modern English.

Initially the root sense was 'to stand by' or 'be present at'; over time the meaning shifted to 'to help or aid', and the noun came to mean 'one who helps' (including the sports sense of 'one who makes an assist').