Old English Tiwesdæg, from Tiwes, genitive of Tiw "Tiu," from Proto-Germanic *Tiwaz "god of the sky," differentiated specifically as Tiu, ancient Germanic god of war, from PIE root *dyeu- "to shine" (see diurnal). Cf. Old Norse tysdagr, Swedish tisdag, Old High German ziestag.
The day name (second element dæg, see day) is a translation of Latin dies Martis (cf. Italian martedi, French Mardi) "Day of Mars," from the Roman god of war, who was identified with Germanic Tiw (though etymologically Tiw is related to Zeus), itself a loan-translation of Greek Areos hemera. In cognate German Dienstag and Dutch Dinsdag, the first element would appear to be Germanic ding, þing "public assembly," but it is now thought to be from Thinxus, one of the names of the war-god in Latin inscriptions.