Hold up your non-dominant hand in a modified "four" handshape, (the thumb is alongside, not across the palm). Hold your dominant hand about five inches away from your non-dominant hand. (The dominant hand is between your non-dominant hand and your chest or face.) Then quickly move your dominant hand forward until it smacks into your non-dominant hand.
JAIL or PRISON:
Another version of this sign that I tend to see used more for jail uses "V" handshapes.
JAIL or PRISON (version)
You might see this version too. For this version the dominant "V" hand moves quickly forward and down slightly until the back of the index and middle fingers slap against the front of the non-dominant hand's index and middle fingers. The dominant hand keeps moving forward and down past the non-dominant hand. I see this version used somewhat more as a verb. ("To be thrown in jail!)
JAIL: (version)
Notes: