- third (adj.) O.E. metathesis of þridda, from P.Gmc. *thridjas (Cf. O.Fris. thredda, O.S. thriddio, M.L.G. drudde, Du. derde, O.H.G. dritto, Ger. dritte, O.N. þriðe, Goth. þridja), from PIE *tritjos (Cf. Skt. trtiyas, Avestan thritya, Gk. tritos, L. tertius, O.C.S. tretiji, Lith. trecias, O.Ir. triss). Related to O.E. þreo (see THREE (Cf. three)). Metathesis of thrid into third is attested from c.950 in Northumbria, but thrid was prevalent up to 16c. The noun meaning "third part of anything" is recorded from late 14c. Third rail in electric railway sense is recorded from 1890. Third World War as a possibility first recorded 1947. Third-rate "of poor quality" is from 1814, ultimately from classification of ships (1640s); third class in railway travel is from 1839. Third Reich (1930) is a partial translation of Ger. drittes Reich (1923). Third party in law, insurance, etc., is from 1818.
Etymology dictionary. 2014.