Whack the slack
The act of slapping your penis on a hard object in order to achieve or sustain an erection, most often followed by masturbation.
Samson: "john, i know you were wanking in there but damn that took a while brah"
John: "well it is common knowledge that you must whack the slack before you can beat the meat"
John: "well it is common knowledge that you must whack the slack before you can beat the meat"