Pugnacity is the noun: the quality of being pugnacious. A person's pugnacity is how ready they are to argue or to fight. Historians write about the pugnacity of a young nation; a teacher might talk about a child's pugnacity in class. It is formal. In ordinary speech people say he is so combative, or she always wants a fight. Though actually, it names the trait, not a person and not a single fight. You have pugnacity; you are not a pugnacity. Same Latin fist-and-fight family as pugnacious and pugnaciously. It is not a boxing-term like pugilism; pugilism is the sport, pugnacity is a temper. The -acity ending is the same kind of noun-ending you hear in audacity.