John Calvin Commentary Genesis 49:9

"Judah is a lion`s whelp; From the prey, my son, thou art gone up: He stooped down, he couched as a lion, And as a lioness; who shall rouse him up?" — Genesis 49:9 (ASV)

Judah is a lion’s whelp. This comparison confirms the previous sentence: that Judah would be formidable to his enemies.

Yet Jacob seems to allude to the reduction in power that occurred when the greater part of the people revolted and joined Jeroboam. For then the king of Judah became like a sleeping lion; he did not shake his mane to spread his terror far and wide but, as it were, lay down in his den.

Yet a certain secret power of God lay hidden beneath that inactivity, and those who most desired his destruction, and who were best able to harm him, did not dare to disturb him. Therefore, after Jacob transferred supreme authority over his brothers to Judah alone; he now adds, by way of correction, that even if Judah's power were diminished, he would nevertheless remain terrible to his enemies, like a lion who lies down in his lair.