1. To begin to experience muscle cramps. Whoa, I need to rest for a minute—my leg is cramping up.

2. To cause one to begin to experience muscle cramps. In this usage, a noun or pronoun can be used between "cramp" and up." I need to get some water—dehydration is starting to cramp me up.

3. To squeeze or force someone or something into an overly small space. In this usage, a noun or pronoun can be used between "cramp" and up." Every time you try to cramp up more stuff in this closet, it all comes crashing down on me the next time I open it. Don't leave the dog cramped up in his cage all day!