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3.
v. when, in a college dormitory bathroom or other communal restroom facility, all of the shower stalls are occupied, and an individual who is short on time (or manners) strips off his clothes and runs into a random stall, gleefully yelling, "showerjack!" and forcing the former occupant out.

n. an instance of the above maneuver.
1) "Showerjack!!"
"Dude!? What the %^&* is wrong with you?"

2) "That was a nasty showerjack; I'm going to go soak his underwear in capsaicin."
by Greevey McSeamus August 29, 2008
 
1.
A little boy that finds out the wonders of masturbation while in the shower. When he gets older he is constantly in the shower for 2 hours at a time.
Jack : why does he take so long in the shower ?

Joe : because he is such a shower jack
by Hsbevebnsjchdbsnelfjbd d July 15, 2013
 
2.
To steal shower first even though another roommate or family member is getting ready for a shower, leaving them to wait impatiently for you in their bathrobe or towel.
Just as Tina prepared to enter the bathroom, she could hear that her roommate had showerjacked, and was stuck waiting for another hour.
by Snoopy Snoopy Poop Dogg April 06, 2011