look up any word, like blumpkin:
 
1.
Pl. n. Two dudes that have had sex with the same chick, not necessarily at the same time.
Mike had relations with Annie, so did Bill. Bill and Mike are now cock brothers.
by Tyrone 667 January 08, 2007
 
2.
One who partakes in intercourse with a lady, who has had intimate relations with a close friend.
Me and me mate are cock brothers 3 times over.
by Joe Mc May 24, 2008