What is the X in X-mas?
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.
from bolt (n.) in its various senses; from a crossbow arrow's quick flight comes the meaning "to spring, to make a quick start" (early 13c.). Via the notion of runaway horses, this came to mean "to leave suddenly" (early 19c.). Meaning "to gulp down food" is from 1794. The meaning "to secure by means of a bolt" is from 1580s. Related: Bolted; bolting.