means A metal ring or fastening, usually one of a linked pair, used to bind the wrists or ankles and prevent movement — or, more loosely, anything that restrains or limits freedom.

from From Old English 'sceacul,' a fetter or bond, with cousins across the old Germanic tongues — Dutch 'schakel' (a link of a chain) and related Scandinavian words. The deep sense is 'something that links or binds,' which is why a shackle can be both the iron on a prisoner and the connecting loop on a chain. The figurative 'shackles of oppression' follows naturally: language reaching for the most physical image of being held.