I'm not a rabid fan, but I've never heard it applied to a pitch. But there are many, many expressions used to describe "specialty" pitches.
I tend to think of the expression as describing flies on which the umpire calls the "infield fly rule," but I guess it also applies to "pop fouls."
Edit. I did have the following pitch in the back of my mind as a possible candidate, but couldn't think of the name. I quote: The rising fastball is a pitch where the ball appears to hop up as much as a third of a meter with a sudden increase in speed.
The infield fly rule is too restrictive, relating also to the number of outs. Let's say, a pop-up is a fly ball which is likely to be in the air for at least three or four seconds and not travel more that a hundred feet or so from the plate - that is, not likely to reach into the outfield by a significant amount.