John Gill 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)

If you have done foolishly in lifting up yourself against a king, against whom there is no rising up; by speaking evil of him, or rebelling against him; which is acting a foolish part, since it brings a man into troubles and difficulties inextricable; or by self-commendation, which is the height of folly, and the fruit of pride; or carried it in such a haughty and overbearing manner to others, as to provoke to wrath and anger;

or if you have thought evil; purposed and designed it, and contrived the scheme of doing it, though not yet put in execution; though folly is not actually committed, yet since the thought of it is vain, care should be taken to prevent it;

[lay] your hand upon your mouth: think again before the thing resolved on is done; as studious and thoughtful men put their hand to their mouth, when they are deeply considering any affair before them: or put a stop to the design, let it go no further; what has been thought of in the mind, let it never come out of the mouth, nor be carried into execution; stifle it in the first motion: or if this respects a foolish action done, as it also may, since it stands connected with both clauses, then the sense is, be silent; do not pretend to deny the action, nor to excuse it; nor to say one word in the defence of it; nor to lay the blame upon others; and much less to calumniate and reproach such who faithfully reprove for it; take shame to yourself in silence, and repent of the iniquity done.

Aben Ezra thinks these words are said to Ithiel and Ucal; but rather, to any and everyone, to all that should hear and read these proverbs. The Targum is, "do not lift up yourself, lest you be foolish; and do not stretch out your hand to your mouth."