A measure of the attractiveness of a woman.
Explanation -- An attempt to have an affair with a smart, beautiful and awesome woman might lead to a downward spiral of divorce, financial losses, and emotional turmoil to the point where you are living in a cardboard box under a bridge. If that were to happen, would you look back on the wreckage your life has become and say, "She was worth it"? If that is the case, she is "Box worthy".
Dave: Danny, Have you met your girlfriends' parents yet?
Danny: Yeah dude, her mom is smoking hot. Totally Box worthy!
Dave: Wait until you see her grandma!