flail / flāl/ • n. a threshing tool consisting of a wooden staff with a short heavy stick swinging from it. ∎ a similar device used as a weapon or for flogging. ∎ a machine for threshing or slashing, with a similar action.• v. 1. wave or swing or cause to wave or swing wildly: [intr.] his arms were flailing helplessly | [tr.] he flailed his arms and drove her away. ∎ [intr.] flounder; struggle uselessly: I was flailing about in the water he flailed around on the snow.2. [tr.] beat; flog: he escorted them, flailing their shoulders with his cane.