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