No rule for this one
No ending or word group predicts the article of Arm. Learn it as a pair and say it out loud: der Arm.
der Arm
It's der Arm. Arm is masculine, plural die Arme.
Mein Arm ist gebrochen.
My arm is broken.
No ending or word group predicts the article of Arm. Learn it as a pair and say it out loud: der Arm.