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.

rare metallic element, 1832, Modern Latin, named by its discoverer, Swedish chemist Jöns Jakob Berzelius (1779-1848) from thorite (silicate of thorium), the name of a mineral found in Norway from which it was extracted (which Berzelius had also named, as thoria, in 1828), and named in honor of the Scandinavian god Thor. With metallic element ending -ium.

fifth day of the week, Middle English Thuresdai, from Old English þurresdæg, a contraction (perhaps influenced by Old Norse þorsdagr) of þunresdæg, "Thor's day," from Þunre, genitive of Þunor "Thor" (see thunder (n.)); from Proto-Germanic *thonaras daga (source also of Old Frisian thunresdei, Middle Dutch donresdach, Dutch donderdag, Old High German Donares tag, German Donnerstag, Danish and Swedish Torsdag "Thursday"), a loan-translation of Latin Jovis dies "day of Jupiter."

Roman Jupiter was identified with the Germanic Thor. The Latin word is the source of Italian giovedi, Old French juesdi, French jeudi, Spanish jueves, and is itself a loan-translation of Greek dios hēmera "the day of Zeus." Holy Thursday in Middle English was Ascension Day (40 days after the Crucifixion; the use of the same phrase for Maundy Thursday, the day before the Crucifixion, is modern and was criticized as incorrect.