'lump-sum' is a compound of 'lump' + 'sum'. 'lump' originates from Middle English, specifically the word 'lump(e)', where it meant 'a mass or clump'; 'sum' originates from Latin, specifically the word 'summa', via Old French 'somme', where it meant 'total'. The compound 'lump-sum' is recorded in English from the early 19th century.

'lump-sum' formed in modern English by combining the words 'lump' and 'sum' (both long-established words) and became used as a fixed term—for example in phrases like 'lump-sum payment'—during the 19th century.

Initially it literally meant 'a sum in a lump (a single mass of money)'; over time it came to mean specifically 'a single full payment of the total amount' as used today.