its another word for "bestie bae" except instead of the word bestie you replace it with breast so then it turns into "breastie bae" it can be used casually in coversation in place of bestie
person 1: omg so true breastiebae!
person 2: the fuck did you just call me
When you put your girl friends before your boyfriend; An alternative to woes before bros.
Jim:"So, Carol, are you going out to dinner with the guys and me tonight?
Carol:"No, sorry Jim"
Jim:"I thought that you said earlier that you could go"
Carol:"Brittany and I are watching Netflix tonight, breasties before testes!