I would like to add my conjecture to what Mr. Quinion said about shaking a stick being a threatening gesture. Back then, gentlemen carried sticks: walking sticks. It was very common to raise them (and shake them) as a threat, or for emphasis. (I can't help but think of the little guy on the Monopoly cards!) If there were too many of the opposition, then presumably you wouldn't want to shake your stick at them.