A combination of the words boob and virginity. Girls loose this when they first let another person ( guy or girl, I dont judge ) grab or feel up their chest. it is sort of like loosing your virginity but only with your chest. Also guys can loose this when they feel or touch thier first set of breasts.
Note: over the bra does not count, ( it can be related to oral sex, you are almost there but not quite. )
Girl 1: Heyy girl, did you have fun on your date
Girl 2: Yes I did, but I got carried away and lost my boobginity to that guy I was with. before I knew it, he was all over my chest
Guy 1: Hey man I saw you out with that girl
again, you guys have a good time???
Guy 2: you dont know the half of it! I totally lost my boobginity with that girl.