The area on a woman's body above which she is not okay with a man shooting their load after pulling out before climax during intercourse. Usually the breasts or upper-chest; a woman's acceptable cumshot boundary.
A verbal play on the mason dixon line that was the dividing line between the Northern and Southern states during the U.S. Civil War.
Man: "She won't ever let me cum on her face."
Friend: "What? Really? What about on her tits?"
Man: "Yeah, she's okay with that- she just tells me to keep it 'below the mason jism line'."