Metal chains or restraints fastened around a person's wrists or ankles to prevent them from escaping, or anything that restricts freedom.
From Old English 'sceacel,' likely related to 'shake.' The metal restraints that literally shake or rattle became the symbol for anything that limits freedom, making the word metaphorical by the 1400s.
Shackles were so strongly associated with slavery and imprisonment that the word became a powerful metaphor—people today talk about 'shackles of poverty' or 'shackles of tradition,' using this image of metal chains to describe any invisible force that traps you.
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