slang for jerk-off session: the event; past, present, or future; where a male has allotted himself several minutes to several hours for nothing other than private, uninterrupted masturbation. A jerk sesh is typically accompanied by one or several forms of visually stimulating material i.e. internet pornography, magazines, movies, etc.
"Goodnight fellas. I'm gonna get in a quick jerk sesh then its off to bed..."