Lay thine hand upon thy mouth — i.e., be silent. Agur deprecates two things which may easily lead to a quarrel, arrogance and malice. He explains this in the next verse.
Related Commentaries of Proverbs 30:32
Proverbs 30:32
32 If thou hast done foolishly in lifting up thyself, or if thou hast thought evil, lay thine hand upon thy mouth.