Old English metathesis of þridda, from Proto-Germanic *thridjas (cf. Old Frisian thredda, Old Saxon thriddio, Middle Low German drudde, Dutch derde, Old High German dritto, German dritte, Old Norse þriðe, Gothic þridja), from PIE *tritjos (cf. Sanskrit trtiyas, Avestan thritya, Greek tritos, Latin tertius, Old Church Slavonic tretiji, Lithuanian trecias, Old Irish triss).
Related to Old English þreo (see 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 German drittes Reich (1923). Third party in law, insurance, etc., is from 1818.
Coming next after second, as in order, rank, or time.
Being the digit that is adjacent to and is on the outermost side of the second digit, as on a foot.