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Cripple stud is a term from framing carpentry. It refers to a stud that is shorter than full length, which for a stud is from floor to ceiling. Cripple studs are used when a large hole is made in a wall for a good and intentional purpose. Such holes are mostly doors or windows.

The best way to grok the cripple stud is visually. Let's say you decide to put a door in a completed wall, so you take a big saw and cut a hole in the wall a few inches bigger than the door on all sides. Because the door is wider than the 16 inches or so between the studs that run from the floor to the ceiling, you end up with one or more short studs that hang down from the ceiling to the top of the hole. Those are cripple studs. Now, if you had cut out a window hole instead of a door hole, you'd have cripple studs both above and below the hole.

Now, as the purpose of the studs in bearing walls is to support what is above them, cutting studs off short weakens the structure and violates the construction code. The solution is to put something under the cripple studs that are now dangling from the ceiling. That something is a horizontal beam of adequate strength to carry the weight that was originally carried by the studs before they were crippled. Cool people know that this horizontal support is called a lintel. The lintel is supported on both sides by another type of short stud called a jack stud. The jack studs are sistered with the full-length king studs on both sides of the opening.

Of course there's more to making a door or window than just cutting holes in walls, but now, at least, you know what a cripple stud is.