The point at which a woman's craziness outweighs her level of beauty.
For a woman to be considered "below the Mendoza Line," she must be sufficiently crazy that it doesn't matter how beautiful she is; you would still not sleep with her.
Based on the baseball term of the same name, referring to the point at which a player's poor batting average cannot be justified despite his defensive abilities.
George- "Dude, Suzy's all over you tonight. Why are you ignoring her?"
Brett- "Trust me man, she's way below the Mendoza Line. Her last boyfriend had to change his social security number."
George- "...mind if I go for it?"