As a dog returneth to his vomit - See note on 2-Peter 2:22.

As a dog returneth to his vomit,.... Who being sick with what he has eaten, casts it up again, and afterwards returns unto it and licks it up;
so a fool returneth to his folly, or "repeats" (a) it, time after time, many times, as Ben Melech; or a wicked man turns to his wickedness, who, having had some qualms upon his conscience for sin, for a while forsakes it; but that fit being over, and he forgetting all his former horror and uneasiness, returns to his old course of life: a wicked man is here compared to a dog, as he is elsewhere for his impudence and voraciousness in sinning; and the filthiness of sin is expressed by the vomit of a dog, than which nothing is more nauseous and loathsome; and the apostasy of the sinner, from an external course of righteousness into open profaneness is signified by the return of this creature to it. This is said to be a "true proverb", 2-Peter 2:22, where it is quoted and applied.
(a) "qui iterat", Tigurine version, Michaelis; "iterans", Montanus, Mercerus, Cocceius, Gejerus; "duplicans", Schultens.

The dog is a loathsome emblem of those sinners who return to their vices, 2-Peter 2:22.

returneth . . . folly--Though disgusting to others, the fool delights in his folly.

The series of proverbs regarding fools is continued:
Like a dog which returneth to his vomit,
Is a fool who cometh again with his folly.
שׁב is like שׁונה, particip.; only if the punctuation were כּכּלב, ought "which returneth to his vomit" to be taken as a relative clause (vid., under Psalm 38:14). Regarding על as designating the terminus quo with verbs of motions, vid., Khler under Malachi. 3:24. On קא = קיא, cf. Proverbs 23:8. Luther rightly; as a dog devours again his vomit. The lxx translate: ὥσπερ κύων ὅταν ἐπέλθῃ ἐπὶ τὸν ἑαυτοῦ ἔμετον; the reference in 2-Peter 2:22 : κύων ἐπιστρέψας ἐπὶ τὸ ἴδιον ἐξέραμα, is thus not from the lxx; the Venet. is not connected with this N.T. citation, but with the lxx, if its accordance with it is not merely accidental. To devour again its vomit is common with the dog.
(Note: Vid., Schulze's Die bibl. Sprichwrter der deutschen Sprache, p. 71f.)
Even so, it is the manner of fools to return again in word and in deed to their past folly (vid., regarding שׁנה with ב of the object. Proverbs 17:9); as an Aram. popular saying has it: the fool always falls back upon his foolish conduct.
(Note: Vid., Wahl's Das Sprichwort der heb.-aram. Literatur, p. 147; Duke's Rabbin. Blumenlese, p. 9.)
He must needs do so, for folly has become to him a second nature; but this "must" ceases when once a divine light shines forth upon him. The lxx has after Proverbs 26:11 a distich which is literally the same as Sir. 4:21.