John Gill Commentary Psalms 39:11

"When thou with rebukes dost correct man for iniquity, Thou makest his beauty to consume away like a moth: Surely every man is vanity. Selah" — Psalms 39:11 (ASV)

When you with rebukes do correct man for iniquity
The psalmist illustrates his own case, before suggested, by the common case and condition of men, when God corrects them; which he has a right to do, as the Father of spirits, and which he does with rebukes; sometimes with rebukes of wrath, with furious rebukes, rebukes in flames of fire, as the men of the world; and sometimes with rebukes of love, the chastenings of a father, as his own dear children; and always for iniquity, whether one or another; and not the iniquity of Adam is here meant, but personal iniquity.

And correction for it is to be understood of some bodily affliction, as the effect of it shows; you make his beauty to consume away like a moth; that is, secretly, suddenly, and at once; as a moth eats a garment, and takes off the beauty of it; or as easily as a moth is crushed between a man's fingers; so the Targum: ``he melts away as a moth, whose body is broken:'' the Vulgate Latin, Septuagint, Ethiopic, and Arabic versions, and so the metaphrase of Apollinarius, read, as a spider which destroys itself. The word rendered "beauty" takes in all that is desirable in man; as his flesh, his strength, his comeliness, his pleasantness of countenance all which are quickly destroyed by a distemper of the body seizing on it;

wherefore the psalmist makes and confirms the conclusion he had made before: surely every man [is] vanity; (See Gill on Psalms 39:5);

Selah; on this word, (See Gill on Psalms 3:2).