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Two men who have shared a particular sexual partner consecutively, although not necessarily in quick succession. The essence is, your Milk Brother was the last person to have slept with this particular partner before you did. (Like blood brothers have shared blood, milk brothers have shared milk, at least symbolically.)
Guy at Party: "Why does Javier's girlfriend get all tense, but also kinda flirty whenever Claude is around?"

Guy 2: "She used to bang Claude. In fact, Claude and Javier are Milk Brothers"
by Purveyor of fine expressions February 25, 2009
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Male friends who have shared sexual exploits (ie. having sexual intercourse with the same woman at the same time.)
Carlos & Diego are milk brothers; when they go out to hook up, they often bring back one chick and both do her.
by JohnnyKi July 15, 2005
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