Mean people are not cool. They go around acting belligerently, or being hostile and aggressive to others. No one likes a meanie.
The word belligerently comes from the Latin belligerant meaning "waging war," which is in effect, what someone acting belligerently is doing. Think of those two l's in the word as two soldiers marching in formation and you might have some help remembering that belligerently is spelled with two l's and not one. Often this word is used to describe the way that some people start to behave when they've had too much to drink, lashing out belligerently, or being overly aggressive.