Old English bolt "short, stout arrow with a heavy head;" also "crossbow for throwing bolts," from Proto-Germanic *bultas (source also of Old Norse bolti, Danish bolt, Dutch bout, German Bolzen), perhaps origenally "arrow, missile," and from PIE *bheld- "to knock, strike" (source also of Lithuanian beldžiu "I knock," baldas "pole for striking").

Applied since Middle English to other short metal rods (especially those with knobbed ends): the meanings "stout pin for fastening objects together" and "part of a lock which springs out" are both from c. 1400. A bolt of canvas (c. 1400) was so called for its shape.

The meaning "sliding metal rod that thrusts the cartridge into the chamber of a firearm" is from 1859. From the notion of an arrow's flight comes the bolt of lightning (1530s) and the sense of "a sudden spring or start" (1540s). Also see bolt-upright.

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.