As a dog returneth to his vomit, so a fool returneth to his folly.
- King James Bible "Authorized Version", Cambridge Edition
New International Version
As a dog returns to its vomit, so fools repeat their 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, also 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. Dogs were considered unclean in Biblical times as they were commonly scavengers of the dead and they appear in the Bible as repugnant creatures, symbolizing evil. The reference to vomit indicates excessive indulgence and so also symbolizes revulsion.
The incorrigible nature of fools is further emphasized in Proverbs 27:22,
"Though you grind a fool in a mortar, grinding them like grain with a pestle, you will not remove their folly from them."
The "fool" spoken of is a person lacking moral behavior or discipline, just as the "wise" person in Proverbs is one who knows to (or knows how to) behave wisely. There is not necessarily any connection to intelligence in the modern sense.
Saint Peter refers to the proverb in his second epistle (2 Peter 2:22), "But it is happened unto them according to the true proverb, The dog is turned to his own vomit again; and the sow that was washed to her wallowing in the mire."
Rudyard Kipling cites this in his poem The Gods of the Copybook Headings as one of several classic examples of repeated folly:
“ As it will be in the future, it was at the birth of Man
There are only four things certain since Social Progress began.
That the Dog returns to his Vomit and the Sow returns to her Mire,
And the burnt Fool's bandaged finger goes wabbling back to the Fire; ”
"The Gods of the Copybook Headings" is a poem published by Rudyard Kipling in 1919, which, editor Andrew Rutherford said, contained "age-old, unfashionable wisdom" that Kipling saw as having been forgotten by society and replaced by "habits of wishful thinking."
The "copybook headings" to which the title refers were proverbs or maxims, extolling virtues such as honesty or fair dealing that were printed at the top of the pages of 19th-century British students' special notebook pages, called copybooks. The school-children had to write them by hand repeatedly down the page. (https://en.wikipedia.org/wiki/The_Gods_of_the_Copybook_Headings)
But it is happened unto them according to the true proverb, The dog is turned to his own vomit again; and the sow that was washed to her wallowing in the mire.