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!"
when a females vagina has large, meaty flaps. usually in a roast beef kind of color. see "beef curtains", "t bone flaps." simular to a second rate fast food chain's large roast beef sandwhich.
"yo, i was with this girl last night.. she had a double serving of the big montana"
1. The largest of the Arby's Food chain Sandwiches.
2. A vagina that is loose and has been beat up like Grace Jones; lips are brown to brownish purple and resemble premium cuts of roast beef from a locally owned deli; sometime accompanied by Horsey Sauce.
That broad had a big montana, but I still ate it all.