User blog comment:LostInRiverview/The Sims Wiki News - 22nd May 2014/@comment-1997588-20140524055430/@comment-1519608-20140524073521

UrbanDictionary's definition is in the right ballpark.

"Obnoxious partying males who are often seen at college parties. When they aren’t making an a** 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."

So a bro is generally a loud, unintelligent guy who forms loud, unintelligent friendships with other loud, unintelligent guys.