Linea Nigra
Linea Nigra is the vertical skid mark left on the chest of a woman after she gets titty fucked. This appears more commonly on smaller breasted women, as the man has to squat lower and spread his legs wider in order to facilitate the procedure. In Obstrestics & Gynecology, Linea Nigra is the name given to the dark pigmented vertical stripe, that sometimes appears on the abdomen of a pregnant woman.
Dude, last night I gave your sister a Linea Nigra and she's not even pregnant!