A.T. Robertson Commentary 2 Peter 2:22

"It has happened unto them according to the true proverb, The dog turning to his own vomit again, and the sow that had washed to wallowing in the mire." — 2 Peter 2:22 (ASV)

It has happened (συμβεβηκεν). Perfect active indicative of συμβαινω, for which see 1 Peter 4:12.

According to the true proverb (το της αληθους παροιμιας). "The word (το used absolutely, the matter of, as in Mt 21:21; James 4:14) of the true proverb" (παροιμια a wayside saying, for which see Joh 10:6; 16:25,29). The first proverb here given comes from Pr 26:11. Εξεραμα is a late and rare word (here only in N.T., in Diosc. and Eustath.) from εξεραω, to vomit.

The sow that had washed (υς λουσαμενη). Hυς, old word for hog, here only in N.T. Participle first aorist direct middle of λουω shows that it is feminine (anarthrous). This second proverb does not occur in the O.T., probably from a Gentile source because about the habit of hogs. Epictetus and other writers moralize on the habit of hogs, having once bathed in a filthy mud-hole, to delight in it.

To wallowing (εις κυλισμον). "To rolling." Late and rare word (from κυλιω, Mr 9:20), here only in N.T.

In the mire (βορβορου). Objective genitive, old word for dung, mire, here only in N.T. J. Rendel Harris (Story of Ahikar, p. LXVII) tells of a story about a hog that went to the bath with people of quality, but on coming out saw a stinking drain and went and rolled himself in it.