The jack-in-the-box is a musical box toy with a handle crank on the side. As the crank is wound, music is played, and tension increases in the handle. The crank becomes more difficult to turn until the clown or jack springs out of the top of the box from beneath the lid.
In tattoo artwork, the jack-in-the-box is shown sprung. But what comes out of the box is anybody's guess rarely is the little emerging puppet a simple clown in tattoo imagery. Sometimes it is a SKULL or any other numerous types of heads. A common head to pop out might be the evil CLOWN with wacky hair, makeup, and bulbous nose, but also a sneer and wicked look about the face. This is not far from the actual purpose of the toy, which is to give a bit of a start or scare as the jack pops out, and these more malevolent versions capitalize on the scary and surprising aspect of this play thing a bit of an unexpected and bad surprise, even if you're familiar with the toy.