It more specifically means someone who you keep bumping into, who pops out of nowhere, usually when you don't want them to. "He's like a jack-in-the-box, he keeps appearing when I least expect it!"

Context would really help here, but basically, a "Jack-in-the-box" is a toy which is a box with a lid and a little figure inside. You wind up the box, with music playing as you do so. At some point, the lid slams open and the figure - usually a clown - pops out. It's meant to surprise. So the expression is meant to compare something to that, something that just "pops out suddenly" and surprises you.

Erm I'd say it probably refers to being surprised or something of the sort? Its not a commonly used expression though, or to my knowledge it isnt lol

u wanna BOB coming, but MIKE coming.