(s)teh₂n-

Root for thunder, yielding Latin tonare, Old English thunor (thunder), Norse Thor.‌​‌​‍​‍​‌​‍​‍​‍​‍​‌​‌​‌​‍​‍​‌​‌​‌​‍​‌​‍​‍​‌​‌​‍​‍​‌​‍​‍​‌​‌​‍​‌

Discussion

The PIE root *(s)teh₂n- (to thunder, to resound) is one of the most dramatically preserved roots in ‌​‌​‍​‍​‌​‍​‍​‍​‍​‌​‌​‌​‍​‍​‌​‌​‌​‍​‌​‍​‍​‌​‌​‍​‍​‌​‍​‍​‌​‌​‍​‌comparative linguistics, surviving as the name of the thunder god in three separate branches — Germanic Thor/Donar, Celtic Taranis, and Latin Jupiter Tonans — and as the common word for thunder across the family.

The s-mobile in the initial position (the parenthetical s-) means the root appeared in two variant forms in PIE itself: *steh₂n- and *teh₂n-, with the *s- apparently optional. This variation is preserved in the daughter languages: Latin tonāre (to thunder, without s-) versus English thunder, stun, and astonish (all with the s- or its reflexes).

Old English þunor (thunder) and the divine name Þunor (the thunder god) continue *teh₂n- with the regular Germanic sound changes: PIE *t > PGmc *þ (Grimm's Law). The Norse form Þórr (Thor) is a shortened form of the same theonym. German Donner (thunder) shows the High German consonant shift (*þ > d), and the personal name Donar was the Continental Germanic name for the same deity. Thursday (OE Þūnresdæg, "Thunder's day") preserves the god's name as a day of the week, calquing Latin diēs Iovis ("Jupiter's day," French jeudi) — both days named after the thunder god of their respective pantheons.

Latin tonāre (to thunder), tonitrus (thunder), and the epithet Iuppiter Tonāns ("Jupiter the Thunderer") continue the root without the s-mobile. The derivatives astonish (from Vulgar Latin *extonāre, "to strike with thunder") and stun (from Old French estoner, same source) both preserve the original violence of the image: to be astonished is, etymologically, to be thunderstruck.

The Celtic branch preserves the theonym: Gaulish Taranis (the thunder god, depicted on coins and mentioned by Lucan) continues *teh₂n- with a different suffix. The Old Irish torann (thunder) provides the common noun.

Sanskrit stanáyati ("it thunders") and the noun stánayitnu- ("thunder") preserve the root with the s-mobile in the Indo-Iranian branch, confirming that both variants (*steh₂n- and *teh₂n-) existed in PIE.

The deification of thunder in at least three independent IE traditions (Norse Þórr, Celtic Taranis, Roman Iuppiter Tonāns) suggests that the PIE-speaking community already associated thunder with divine power, even if no single "PIE thunder god" can be reconstructed with certainty. The word itself — violent, onomatopoeic, rooted in the sound of the sky splitting — may be as old as human language about weather.

Notes

Pokorny 1021. English thunder, Norse Thorr, Latin tonitrus.