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.
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 (kaikki extract in evidence packet), source_ref kaikki:3f6f69a190036ae4:439002998 (etymology 1) — Middle English shakel, Old English sceacul; the link to *skakaną (to shake) is recorded there as doubted by the Oxford English Dictionary.
Three branches
扣在手腳上的金屬環
這一組就是核心畫面本身：名詞是那個環，動詞是把環扣上去的動作，兩個合起來才是完整的畫面。
noun · B2
One of a pair of metal rings joined by a chain, locked around a person's wrists or ankles so that they cannot move easily or get away. The word is also used for a similar restraint put on an animal.
腳鐐／手銬
把東西扣在一起的金屬扣具
同一個形狀，換了用途。U 型的環用插銷橫著鎖住，扣的不再是人的手腳，而是兩樣需要連在一起的東西。圈比穿過去的東西大，所以接起來之後還能轉、還能晃——這正是繩索、鐵鏈和掛鎖上會用到它的原因。
noun · C1
A U-shaped piece of metal closed across the open ends by a pin or bolt, used to join two things together while still letting them move a little. The curved metal loop of a padlock is also called the shackle, and on boats shackles join lengths of chain or rope.
卸扣／鎖梁
看不見的枷鎖（比喻）
把腳鐐的畫面搬到沒有金屬的地方。規定、貧窮、傳統、預算扣在人或組織身上，東西還在運作，但只能走到限制允許的範圍。用了這個字就等於表明立場：說話的人認為那些限制太重、不該存在。這是畫面上的連結，不是在說這個意思在歷史上比較晚出現。
noun · C1
The shackles of something are the limits that it puts on your freedom and your happiness, most often a system of government, a set of rules, or a tradition. Saying it shows that you think those limits are wrong.
……的枷鎖