Literally, a masturbating or, "jack-off", marathon.
1. Any large amount of time (hours, days, etc.) solely dedicated to masturbating
vigorously multiple times, with friends/girlfriends, or alone for an extended period of time
Jack's wife had gone to her parents' house for the weekend. Since he had ordered the Playboy channel and didn't want all that hand moisturizer to go to waste, he decided to plop down in front of the TV and have himself a jackoffathon.