A spazzy arm, where your arm is crooked and your wrist is bent so that your hand points downwards and cannot leave your side. Spastics are the most well-known dinosaur armers.
John has a dinosaur arm. Because of it he has earned the nickname Tyrannosaurus John. Watching him try to peel an orange is too funny.
It is what you call an arm when you are cuddling and there is no room for it. The arm is typically bent at the elbow and placed uncomfortably between two bodies. This can also be likened as a T-Rex arm or awkward arm.
"I've gotta move; I'm totally getting dinosaur arm."