If thou hast done foolishly in lifting up thyself, or if thou hast thought evil, lay thine hand upon thy mouth.
If thou hast done foolishly (or, hast been foolish) in lifting up thyself - through pride and insolence. The following clause --
If thou hast thought evil - or hast devised evil, implies that the action supposed is one begun, not completed.
(Lay) thine hand upon thy mouth - (Job 21:5; Job 40:4.) Abstain from all excuse or defense of the past; and curb thyself in silence from speaking and still more from going on to do that evil which thou thoughtest.