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.
From the word’s history
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)
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