Therefore I will not refrain my mouth; rather, I moreover, will not refrain my lips; that is, "You may do as you like under affliction, I claim the right of complaining." Job has already pointed out that nature teaches the animals to complain when they suffer (Job 6:5).
Why, then, should not he? Complaint is not necessarily murmuring; it is sometimes merely expostulation, which God allows (comp. Psalms 4:2; Psalms 77:3; Psalms 142:2, etc.). I will speak in the anguish of my spirit; I will complain in the bitterness of my soul.
Extreme "anguish" and "bitter" suffering excuse complaints that would otherwise be, blare-able (comp. Job 6:2-4).