Old English bolt "short, stout arrow with a heavy head;" also "crossbow for throwing bolts," from Proto-Germanic *bultas (cf. Old Norse bolti, Danish bolt, Dutch bout, German Bolzen), perhaps from PIE root *bheld- "to knock, strike" (cf. Lithuanian beldu "I knock," baldas "pole for striking").
Applied since Middle English to other short metal rods (especially those with knobbed ends). From the notion of an arrow's flight comes the lightning bolt (1530s). A bolt of canvas (c.1400) was so called for its shape. Adverbial phrase bolt upright is from late 14c.