Well, a sling can whip a small stone hard enough to do serious damage. Otherwise, look into the cosh / blackjack / billy club style clubs. Light enough to swing

Well, a sling can whip a small stone hard enough to do serious damage. Otherwise, look into the cosh / blackjack / billy club style clubs. Light enough to swing at a high speed, small enough to hide under loose clothing, with just enough weight at the end to cause deep impact trauma. Anything too light means that the force is dispersed in soft tissue, leaving surface bruises. That added inertia of some extra weight at the end causes the energy of the swing to transfer deeper to bone. I doubt you'll get a scientifically derived number of any certainty, but I'd hazard a guess at two pounds or so at a minimum, and that would need some careful aim to be debilitating.