Albert Barnes Commentary Proverbs 30:32

"If thou hast done foolishly in lifting up thyself, Or if thou hast thought evil, [Lay] thy hand upon thy mouth." — Proverbs 30:32 (ASV)

Lay thine hand upon thy mouth - The act expresses the silence of humiliation and repentance after the sin has been committed, and that of self-restraint, which checks the haughty or malignant thought before it has passed even into words.