Tunnel brothers are 2 men who have had a common female sex partner in their past.
Gerald is currently digging out Muriel, who used to give it up to Asa. Gerald and Asa are now tunnel brothers.
by Straylight990 February 10, 2010
1 more definition
A guy who has banged the same girl you have
Oh, you banged Sally? You know that you and Kyle are tunnel brothers now, right?
by caveman pleasures September 02, 2013