thirdyoudaoicibaDictYouDict[third 词源字典]
third: [OE] Third goes back ultimately to an Indo- European *tritjos, an ordinal derivative of the base that produced English three. Amongst its other descendants were Greek trítos, Latin tertius (source of English tertiary [16]), Russian tretij, Polish trzeci, Latvian trešais, and Welsh tryddydd. In prehistoric Germanic it became *thrithjaz, which has evolved into German dritte, Dutch derde, Swedish and Danish tredje, and English third.
=> three[third etymology, third origin, 英语词源]
third (adj.)youdaoicibaDictYouDict
late Old English metathesis of þridda, from Proto-Germanic *thridja- (cognates: Old Frisian thredda, Old Saxon thriddio, Middle Low German drudde, Dutch derde, Old High German dritto, German dritte, Old Norse þriðe, Danish tredie, Swedish tredje, Gothic þridja), from PIE *tri-tyo- (cognates: Sanskrit trtiyas, Avestan thritya, Greek tritos, Latin tertius (source of Italian terzo, Spanish tercio, French tiers), Old Church Slavonic tretiji, Lithuanian trecias, Old Irish triss, Welsh tryde), suffixed form of root *trei- (see three).

Metathesis of thrid into third is attested from c.950 in Northumbrian, but overall 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.