Rashi: It is a staff of pride, like Pharaoh, who said "Mi Hashem Asher Eshma b'Kolo" (Shemos 5:2).

R. Yonah: It is a branch, like "v'Yatza Choter mi'Geza Yishai" (Yeshayah 11:1). The root of pride is in the Kesil's heart, and the branch is in his mouth. You cannot have a branch that does not bear fruits; the fruits are words of pride and scorn. The Kesil conducts with pride with his mouth and heart; he cannot veer from it. He cannot guard his lips from words of pride, as if the branch of pride is in his mouth, and it must give its fruits.

Malbim: The Evvil is unsure about the laws of Chachmah. There is no proof for them. They are not known via human Sechel; they are from Hashem. One who is not haughty, and does not cross the bounds of his Sechel, to say that his Sechel can understand everything, he is submissive to the laws of Chachmah given from Hashem and guards them. The Evvil speaks opposite to the way of Chachmah, as if his folly is Chachmah. This is due to his pride. His pride sticks out of his mouth like a hump jutting out from a tree.

Rashi: This is like Tishmerem - it guards Chachamim. R. Yonah - This is in place of Tishmerenu Osam. Therefore the Mem has a Shuruk, in place of a Cholem.

R. Yonah: It is not enough for Chachamim that their lips are in their Reshus, to stop them from speaking haughtily. Even more, their lips guard them - when pride comes on their heart, they do not express it with their lips. This is even if they do not think to muzzle their lips, for they accustomed their lips to answer softly, and [say] words of humility, to the point that they cannot speak grandly. Above mentioned an intermediate Midah - "Notzer Piv Shomer Nafsho" (13:3).

Malbim: Lips represent external speech. Regarding Chachamim, it hints that the laws of Chachmah became like something they know with Da'as. This guards them from pride, for they are submissive to the supreme Chachmah and believe in it, "Ki Hashem Yiten Chachmah" (2:6).