Answer:
Hyperbole
Step-by-step explanation:
Hyperbole is an exaggeration that is meant to bring out a point or add humor to the speech. In the sentence above, the reader knows quite well that it is impossible for a person to shake off his arm. However, the exaggeration was clearly used to stress the fact that Fred shook his arm so vigorously.
The intensity was so much that the speaker wondered how Fred did not shake his arm off.