Obnoxious partying males who are often seen at college parties. When they aren’t making an ass of themselves they usually just stand around holding a red plastic cup waiting for something exciting to happen so they can scream something that demonstrates how much they enjoy partying. Nearly everyone in a fraternity is a bro but there are also many bros who are not in a fraternity. They often wear a rugby shirt and a baseball cap. It is not uncommon for them to have spiked hair with frosted tips.
Bros actually chose this name for themselves as they often refer to each other as "bro" even though they are not related.
I couldn't go to sleep last night because some bros at the party next door kept screaming, "Whoooooo!!! YEAAHHHHH! Whooooooo!"