All kinds of weapons, small or big, cold or fire, whether used for self defense or mass destruction are strictly prohibited onboard all flights, all airlines, in all countries and airports.
If you managed to get it onboard a flight, then it must be due to improper security check at the gate, not because they allow it. From the TSA's page on prohibited items onboard:
Item -------- Carry-on? ---------- Checked?
Kubatons -------NO ----------------- OK
And from the European Commission on the same subject:
c. objects with a sharp point or sharp edge (objects with a sharp point or sharp edge
capable of being used to cause serious injury)...
e. e. blunt instruments (objects capable of being used to cause serious injury when used to hit)...
Also, another interesting note in the official UK page
Airport security staff won’t let anything through that they consider dangerous - even if it’s normally allowed in hand luggage.
And in another official UK page:
You can't take any objects in your hand baggage that could cause injury to yourself and other passengers. You may be able to take some of these items in your hold baggage (the luggage you check in) instead.
Just open any official hand luggage restriction page, and you will find the same thing. In short, any object, whether it is called a weapon or not, that can be used to attack people and cause harm to others is prohibited once it is spotted. To be in the safe side and to avoid confiscation or even worse actions, put it in the checked in luggage.
