'third-last' originates from Modern English as a compound of the ordinal 'third' and the adjective 'last'. 'Third' ultimately comes from Old English 'þridda' (from Proto-Germanic roots meaning 'one of three'), and 'last' comes from Old English 'lǣst' meaning 'hindmost' or 'final'.

'third-last' developed from earlier periphrastic phrases such as 'third from the last' or 'third from the end' and later formed into the hyphenated compound 'third-last' in Modern English. A more formal single-word equivalent is the Latin-derived 'antepenultimate' (from Latin 'ante-' + 'penultimus').

Originally speakers expressed the idea with phrases like 'third from the last'; over time the notion was regularized into the compound adjective/noun 'third-last' without a substantial change in meaning (still 'third from the end').