You can picture one object behind every use of this word: two sticks joined by a loose chain. The hand holds the long one; the short, heavy one hangs free at the far end. Because the joint is loose, the far end does not stay in line with your arm — it keeps going after your hand stops, swings out wide, and comes down much harder than the arm alone could bring it. Hold that picture and the rest follows: the thing itself, the work it does, and the way anything moves when it is joined loosely and swinging free.
Wiktionary traces flail to Middle English flayle and Old English fligel, alongside Dutch vlegel and German Flegel, all naming the threshing tool. An older root meaning 'to whip, beat' is suggested but the source itself calls the origin uncertain, so this is a possible line, not a settled one.
依 Wiktionary，flail 來自中古英語 flayle、古英語 fligel，跟荷蘭文 vlegel、德文 Flegel 同源，指的都是打穀的連枷。再往前有人推到一個「抽打」的字根，但來源本身就標為不確定，所以這條線只能當可能的說法，不能當定論。
Wiktionary 'flail' etymology, supplied in packet (kaikki:3f6f69a190036ae4:896567559)
Three branches
那樣東西：鬆鏈接著的甩擊物
這一組就是核心畫面本身，只是換了場合：穀場上的農具、戰場上的兵器、機器上轉動的刀片。三者的構造完全一樣，都是「柄＋鬆接頭＋會甩的重頭」。
noun · C2
A farm tool for beating grain: a long handle with a shorter, heavier stick hanging from the end of it on a chain or a strap, so the short stick swings down hard on the cut plants and knocks the seeds loose.
連枷（打穀用的農具）
那件工作：把東西一下一下打下去
把工具變成動作。人還是站在那裡揮長棍，只是被打的東西從穀物擴到別的東西上；句子裡通常會有受詞。
verb · C2
To beat cut grain with a flail so that the seeds come away from the stalks; to thresh by hand.
用連枷打穀
那個動作：鬆掉了、自己亂甩
這裡借的不是工具，而是工具末端那種「鬆掉、收不住」的甩法：手腳像掛在鏈子上一樣自己甩出去。這是今天最常用的意思。
verb · C1
To swing your arms or legs about in a wide, loose way that you are not controlling, for example when you slip, panic, or fight to keep your balance.
（手腳）亂揮亂甩