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3.
A penis that looks average until it is aroused and then erects to a size well above the norm.
Andrew: Yes, my penis looks only about an inch long when it is soft.

Slampiece: Well I must admit, your Jack in a Box was a nice surprise!
by Quagmire517 September 09, 2011
 
1.
When you have an erection that your girlfriend doesn't know about and you whip it out of your pants and show her your hard on, usually with a really pleased expression on your face.
I was lying in bed reading the other day when Rob pulled down the sheets and sprung his jack in a box on me!
by Robertion August 17, 2011
 
2.
A toy. Not to be confused with "Jack in the box", a fast-food restaurant.
Baby Hugo cried whenever he played with his Jack-In-A-Box. Every time Jack popped out it frightened him. Bad Jack!
by MTF January 22, 2009