From Old English 'þūnresdæg,' meaning 'day of Þunor,' the Anglo-Saxon god of thunder. Þunor (Old Norse Þórr, or Thor) was identified with the Roman god Jupiter through interpretatio germanica, making Thursday the Germanictranslation of Latin 'Iovis diēs' (day of Jupiter). Both deities wielded thunder and lightning as their primary weapons and served as protectors
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The equation of Thor with Jupiter that gives us Thursday = Iovis diēs is not just functional — both gods' names trace back to weather phenomena: Thor from PIE *(s)tenh₂- (to thunder) and Jupiter from PIE *Dyēu-pəter (Sky Father), though they come from different roots entirely.