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1.
A deeply rooted fear that one of your "bros" or you, your bro-self, might be gay.
Dave: "Holy shit Kyle, why do you have gay magazines?!?!"

Kyle: "Dude it's for style tips, the chicks dig gay dudes!"

Dave: "Whatever bro, that's so gay!"

(Dave is suffering from Bromophobia, and he may even be gay himself, as many bros are)
by David A A December 09, 2008
 
2.
When someone is afraid of or threatened by a friendship they see between two male friends.
Sandra won't let Andrew come out tonight. That girl totally has bromophobia.

OR

Charles mad that Andrew is going to the movies with Ross tonight. That guy totally has bromophobia.
by female bromancer December 27, 2010
 
3.
The fear of bros, usually occurring when you realize all bros are secretly gay and then you're thus a homophobe as well
Rajiv: Me and my bros like to give each other blowjobs, but Ricky doesn't like it

Mort: How come?

Rajiv: because Ricky has bromophobia!
by Mr. FuckOff December 22, 2009
 
4.
the mental illness that pertains to someone who is scared of bromosexuality (the act of being as close as possible to another man without homosexuality)
Zach doesnt like to embrace his bromosexuality. maybe he has bromophobia

Other usage:
Zach is a bromophob
Zach is bromophobic
by Brohemian Rhapsody February 09, 2009