The psalmist’s state of mind (39:2–4). The psalmist’s opening words are addressed to himself; within his own mind, he expresses his determination to keep quiet, come what may. Aware that he already has sufficient trouble for one person, he determines not to compound his problems by saying anything evil; so, in the metaphor, he “muzzles” himself.

Peter C. Craigie and Marvin Tate, Psalms 1-50, Volume 19: Second Edition (Word Biblical Commentary), 308.