Pugnacious means ready for a fight. If you're a first year teacher, you probably don't know how to deal with the pugnacious kids in every class. Learn some discipline or they'll end up fighting you, or each other.
Your brother is a pugnacious thug––always ready to use his fists to settle arguments, and he has the strength to do so. That’s the literal sense of pugnacious. You can use pugnacious figuratively, too. When two candidates face off in a debate during a close election, one or the other might be pugnacious. He looks to pick a fight with his opponent and is willing to say almost anything, no matter how outrageous, to make his opponent look bad.