Rundown is the jizz that runs, or flows down the shaft of an erected penis upon ejaculation. It refers to all jizz that makes its way down the shaft, whether from sex with a tight vagina, or from masterbation.
After Mike ejaculated from Michelle's hand-job, the rundown came flowing down his penis.
Horny sub-species of human often seen in a corner with his cock in his hand. Loves to be around women and is heavily attracted to the smell of fish. Likes hairy things. May be gay.