'complainant' originates from Old French, specifically the word 'complaignant', where 'com-' meant 'together' and 'plaignant' meant 'lamenting'.

'complaignant' transformed into the Middle English word 'compleynant', and eventually became the modern English word 'complainant'.

Initially, it meant 'one who laments or grieves', but over time it evolved into its current meaning of 'a person who makes a formal complaint in a court of law'.