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!
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!
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!