"bolt which passes through the eyes of a shackle," 1680s, from shackle (n.), which has been used specifically of the bar of a padlock from mid-14c., + bolt (n.).
Entries linking to shacklebolt
shackle n.
kind of fetter, especially for the wrist or ankle of a prisoner, Middle English shakel, from Old English sceacel, sceacul "shackle, fetter," probably also in a general sense "a link or ring of a chain," from Proto-Germanic *skakula- (source also of Middle Dutch, Dutch schakel "link of a chain, ring of a net," Old Norse skökull "pole of a carriage"), of uncertain origin. According to OED, the common notion of "something to fasten or attach" makes a connection with shake unlikely. Figurative sense of "anything which hinders or restrains" is by early 13c. Related: Shackledom "marriage" (1771); shacklebone "the wrist" (1570s) is Scottish or northern dialect.
bolt n.
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 originally "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). Adverbial phrase bolt upright (like a bolt or arrow) is from late 14c.