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Old English sceacel "shackle, fetter," probably also in a general sense "a link or ring of a chain," from Proto-Germanic *skakula- (cf. 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 use from early 13c. Related: Shackledom "marriage" (1771); shackle-bone "the wrist" (1570s).
mid-15c., from shackle (n.). Figurative use from 1560s. Related: Shackled; shackling.