Ibn Ezra

Tremble (רִגְזוּ - rigzu): Because they shamed him instead of [giving him] his glory, and spoke falsehood concerning him [v. 3], he said: Tremble (Rigzu) out of [fear of] HaShem; fear (pachadu). Similar [is the usage of the root in (Genesis 45:24)]: "Do not tremble/quarrel (al tirg'zu) on the way." [The reason for fear is] because the king takes note of the mockery directed at him. And some say (v'yesh omrim): [That rigzu means "be angry," and the verse means:] Even if you think falsehood in your hearts, let it not come out of your mouths. But this is not correct (ve-zeh eineno nakhon). For it is written (Psalm 15:2): "who speaks truth in his heart," [and (Proverbs 6:18)]: "a heart that devises wicked thoughts (machshevot aven)." The speech of the heart is the essential thing (ha-ikkar), and the tongue is not, but rather [it is merely] an interpreter (melitz) between the heart and the hearers. Rather, the interpretation is: That you should think in your hearts at night upon your beds—when you have no involvement with other people—so that you will not speak falsehood during the day. And this [leads to the final phrase]: "And be silent/still. Selah" (וְדֹמּוּ סֶלָה - Ve-dommu selah).