1) to forcefully enter a building.
2) to work on an object or article of clothing to make it less new (break IT in).
3) to have sex with a virgin (break HER in).
When the Tri Delts got to McDonald's too late and it was closed, they realized they would have to either break in or face the terrible possibility of not getting their favorite food in the world, the triple XXL double cheese bacon burger.
Banana Republic salesman: "Now this outfit would look totally cute on you, but it may take some time to break in."
Josh Hartnett: "I'll break you in."
Banana Republic salesman: "Oh yeah big boy, I'll meet you in the dressing room in 5 minutes."
Rob: "Congratulations on the wedding, buddy."
Brad: "Thanks. I'm so happy Sally stayed a virgin for me all these years."
Rob: "Hate to break it to you, but that's bullshit. I broke the bitch in a few months ago when she was passed out at Jeff's party."