'litigant' originates from Latin, specifically the word 'litigare' (or from the noun 'lis, litis'), where 'lis / litis' meant 'lawsuit' and 'litigare' meant 'to dispute or sue'.

'litigant' changed from Latin 'litigare' into Old French/Anglo-Norman forms (e.g. 'litiger') and entered Middle English as 'litigaten'/'litigate', eventually giving the modern English noun 'litigant'.

Initially it meant 'one who disputes' or 'one engaged in a dispute'; over time it came to mean specifically 'a party to a legal action' (a party in a lawsuit).