The story
From flail + tank; From the flail chains attached to the rotating drum attached to the tank.
Etymology from Wiktionary, CC BY-SA 4.0.
flail tank
Today it means: a type of armored tracked fighting vehicle with a motorized spinning drum with the longitudinal axis of the drum arranged horizontally, perpendicular to the motion of the tank, with flail chains attached to the front end, which can clear landmines and barbed wire by beating them to bits Full definition →