No rule for this one
No ending or word group predicts the article of Fuß. Learn it as a pair and say it out loud: der Fuß.
der Fuß
It's der Fuß. Fuß is masculine, plural die Füße.
Mein Fuß tut weh.
My foot hurts.
No ending or word group predicts the article of Fuß. Learn it as a pair and say it out loud: der Fuß.