As a dog returns to his vomit, so a fool repeats his folly

"As a dog returns to his vomit, so a fool repeats his folly" is an aphorism which appears in the Book of Proverbs in the Bible — Proverbs 26:11 (Hebrew: כְּ֭כֶלֶב שָׁ֣ב עַל־קֵאֹ֑ו כְּ֝סִ֗יל שֹׁונֶ֥ה בְאִוַּלְתֹּֽו Kəḵeleḇ šāḇ ‘al-qê’ōw; kəsîl, šōwneh ḇə’iwwaltōw.), also partially quoted in the New Testament, 2 Peter 2:22. It means that fools are stubbornly inflexible and this is illustrated with the repulsive simile of the dog that eats its vomit again, even though this may be poisonous.

Source: Wikipedia — As a dog returns to his vomit, so a fool repeats his folly (CC BY-SA 4.0)