The freed prisoner had his heavy chains removed and was finally unshackled.

unshackled 是动词 unshackle 的过去分词形式。unshackle 由前缀 un-（表示"去除"）和名词 shackle（镣铐）组成，字面意思是"去掉镣铐"，引申为"解除束缚、获得自由"。在句中表示囚犯被解除了脚镣的状态。