Como dizer "cassetete" em inglês

It´s curious, Thomas. In Brazil those blackjacks being translated as "cassetetes", albeit "cassetete" from the original French would mean something like "skull crackers". So, it could be any of them.
Cassetetes in Brazil are not actually associated with "blackjacks", as far as I know. They wouldn´t be small in lenght and, say, capable of being hidden (in a pocket or the clothes).
Blackjacks and saps were used by the police in America, some cops that were martial arts aficionados had them introduced to the service, here there wasn´t the case. Indeed, the police here would deem them as cold weapons, and somebody found with one would risk staying in the clink for a night along with the blackjack (unintentionally ) being parted from him.
But martial arts would know them, I think, and would find them similar to the "chigiriki" (a stick and chain weapon used by the Japanese martial artists).
In Portuguese, I would think of it as a "bastão" (com correntes, geralmente) or "bastãozinho" but not "cassetete" but they have registered the former on Wordreference, perhaps out of convenience and quick reference.

And Magnus, you are right. Cassetete comes from French - casse-tête, hence it will be "cassetete" in Portuguese. The mistake happens because people makes an analogy (parallel reasoning) with cacete (bâton).
Again, different things, even in French since cacete is bâton, and casse-tete (casser - quebrar, tete - cabeça) .Hence the latter meaning something like "head cracker/smasher" (golpeador de cabeça/esmagador de cabeça), etc.
Thus, "cassetete" doesn´t comes from "cacete" as many of us could think, I myself thought that at first.

If anyone is interested, this website will show how the sap and blackjack dffer from a baton, club, cosh, etc.