1. Friend; commonly used in greetings.
What’s up bro?
I vouch for Todd, we’ve been bros since way back.
2. An alpha male idiot. This is the derogatory sense of the word (common usage in the western US): white, 16-25 years old, inarticulate, belligerent, talks about nothing but chicks and beer, drives a jacked up truck that’s plastered with stickers, has rich dad that owns a dealership or construction business and constantly tells this to chicks at parties, is into extreme sports that might be fun to do but are uncool to claim (wakeboarding, dirt biking, lacrosse), identifies excessively with brand names, spends a female amount of money on clothes and obsesses over his appearance to a degree that is not socially acceptable for a heterosexual male. The female equivalent of the Bro is the Bro Hoe
. Bro Hoes are Bro groupies that hang around bros, many of whom are actually quite hot and are thus spared the scorn that is heaped on Bros.
My sisters new boyfriend is an idiot; the dude is a total bro.