Two men who have shared a particular sexual partner consecutively, although
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
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"