Origin and history of tritium

"next after the second; an ordinal numeral; being one of three equal parts into which a whole is regarded as divided;" late Old English metathesis of þridda, from Proto-Germanic *thridja- (source also of 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).

This is reconstructed to be from PIE *tri-tyo- (source also of Sanskrit trtiyas, Avestan thritya, Greek tritos, Latin tertius (source of Italian terzo, Spanish tercio, French tiers), Old Church Slavonic tretiji, Lithuanian trečias, Old Irish triss, Welsh tryde), suffixed form of root *trei- (see three).

Metathesis of thrid into third is attested from c.950 in Northumbrian (compare wright), but thrid persisted through Middle English and up to 16c. Related: Thirdly.

In 20c. often with a suggestion of "alternative to a seemingly dualistic choice, situation, or problem" (third way, third stream, etc.).

The noun meaning "third part of anything" is recorded from late 14c. Other old words for it were thirding and thirdendeal.

Third rail in electric railway sense is recorded from 1890. Third World War as a possibility is attested by 1947. Third-rate "of poor quality, of distinctly inferior rank or grade" is from 1814, ultimately from classification of ships (1640s); third class of railway travel accommodations is from 1839. Third Reich (1930) is a partial translation of German drittes Reich (1923). Third party in law, insurance, etc., is from 1818.