Picture a ring of metal closed around an ankle, with a short chain running to a second ring on the other ankle. It is not tight enough to stop the leg moving. It is tight enough that it will not come off. The person can stand, and can walk, but only in short steps, and never fast enough to get away. That is the whole idea in one image: movement is still possible, but only as far as the metal allows. Every other use of the word keeps that shape. Rules and costs that hold a business back are the same ring on something that should be moving forward. The U-shaped metal loop that joins a rope to a car, or passes through a gate on a padlock, is the same ring doing a job instead of holding a person.
From the word’s history
The Wiktionary entry in the packet takes shackle back to Old English sceacul and mentions a possible link to a root meaning to shake, while noting that the Oxford English Dictionary doubts it, because that root does not carry the idea of fastening. So treat the sound link as unsettled. What the record does show is that the word has named a fastening ring for over a thousand years, which is why both the prisoner's ring and the sailor's connector sit comfortably under one picture.
資料包裡的 Wiktionary 把 shackle 上溯到古英語 sceacul，並提到可能跟一個意思是「搖動」的字根有關，但同時註明牛津英語辭典對此存疑，因為那個字根沒有「扣住」的意思。所以這條聲音上的線索先當作沒有定論。可以確定的是，這個字指「扣住東西的環」已經超過一千年，這也說明為什麼囚犯腳上的環和水手用的連接扣能放在同一個畫面底下。
Wiktionary, "shackle" (etymology 1)
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