'complainant' originates from Middle English, specifically the word 'compleynant,' where 'compleyn' meant 'to lament or express grief.'

'compleynant' changed from the Old French word 'complaignant' and eventually became the modern English word 'complainant'.

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