Origin and history of Thursday

thunder-god of Norse mythology, Odin's eldest son, strongest of the gods and not the wisest, c. 1020, from Old Norse Þorr, literally "thunder," from *þunroz, related to Old English þunor (see thunder (n.)). His weapon was the hammer Mjölnir (said to be literally "crusher" but there are other theories).

Middle English thonder "sudden or rumbling loud noise which follows a flash of lightning," from Old English þunor "thunder, thunderclap; the god Thor," from Proto-Germanic *thunraz (source also of Old Norse þorr, Old Frisian thuner, Middle Dutch donre, Dutch donder, Old High German donar, German Donner "thunder"), from PIE *(s)tene- "to resound, thunder" (source also of Sanskrit tanayitnuh "thundering," Persian tundar "thunder," Latin tonare "to thunder"). Swedish tordön is literally "Thor's din." Since 18c. explained as due to sudden disturbance of the air caused by a discharge of electricity.

The unetymological -d- also is found in Dutch and Icelandic versions of the word (see D). Of any loud, resounding noise or awful or startling threat or denunciation by 1590s. In mild oaths (by thunder) by 1709; as an intensifier (like thunder) by 1826. To steal (one's) thunder "use the ideas, rhetoric, etc. of one's opponent to one's own advantage" is by 1838, from the theatrical anecdote (attested by 1821) of Dennis's ire at seeing his stage-effect thunder used in another's production. Thunder-stick, an imagined word used by primitive peoples for "gun," attested from 1904.