[Middle English, from Old English thūres dæg, alteration (influenced by Old Norse thōrsdagr, Thor's day) of thunres dæg, Thor's day (translation of Late Latin Iovis diēs, Jupiter's day) : thunres, genitive of thunor, thunder; see (s)tenə- in the Appendix of Indo-European roots + dæg, day; see DAY.]