No rule for this one
No ending or word group predicts the article of Lamm. Learn it as a pair and say it out loud: der Lamm.
der Lamm
It's der Lamm. Lamm is masculine.
Ich esse gern Lamm.
I like eating lamb.
No ending or word group predicts the article of Lamm. Learn it as a pair and say it out loud: der Lamm.