'petitioner' originates from Latin, specifically the word 'petitio' (from 'petere'), where 'petere' meant 'to seek' or 'to request'.

'petitio' passed into Old French as 'peticion' and Middle English as 'petition', and the agentive form with the suffix '-er' produced the modern English word 'petitioner'.

Initially, it referred to the act of 'seeking' or 'requesting'; over time it came to denote the person who makes that formal request ('a petitioner').