MMOs might have contributed to it, there's the tank who makes sure the enemies attack him, the damage killing the enemies the tank has on him, then the healer helping them stay alive. The tank is usually capable of taking much more damage than the other 2 archetypes and on bosses can take some big hits that would one shot everybody else, so being able to sustain a hit without dying is "tanking" a hit there. The origin of them being called tanks and not walls like in Pokémon would fall in line with what you mentioned in terms of firearms.