Big Montana is a derogatory term in regards to when a girl who is a whore has so much vaginal penetration and wear n' tear, that she has vaginal folds that make her box look like an Arby's Big Montana sandwich.
The abbreviation "BM" means the same thing, but is normally used in order to get some strange at the bar or get a drunk chick to show you her vagina.
Example 1: "Fellas, that whore I hooked up with last night looked like she could have been a model for Arby's. All she had to do is spread her legs and show you her Big Montana and boom, she would have been the pinup for a #4 on the Arby's menu board."
Example 2: "Show me your fucking "BM" you cum drenched whore!"