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"