It’s National Poetry Month, so let us praise the humble limerick, in spite of (or perhaps because of) its bawdy, silly rhymes. After all, it’s the only literary form to encompass the poetic genius of both St. Thomas Aquinas and Krusty the Clown from “The Simpsons,” who starts but never finishes the ditty, “There once was a man named Enis…”

Most people know the limerick’s rigid meter and rhyme scheme—the first, second and fifth lines should rhyme with each other, as should the shorter third and fourth lines. But no one really knows where the limerick began or why it’s named for a small Irish city rather than for Peru or Tobago, home to many an Old Man and Young Lady featured in said poems.

Some literary experts look to the ancient Greek comedic playwright Aristophanes, who was fond of writing five-line verses about men or women from Sybaris, a Sicilian city once known for its wealth, who came to absurd ends. Though hardly a bundle of ribald delight, a five-line prayer poem written in Latin by Aquinas (circa 1225-1274) begins, in English, “May my sins be eliminated” and may have been a forerunner of the limerick, since it follows the poem’s AABBA rhyme scheme.

By the 14th century the first unequivocal limerick made its appearance. While Petrarch was writing sonnets to his love Laura, an anonymous British bard was musing about a lion: “whether he play / or take his prey / he cannot help but slay.”

For a moment the limerick threatened to turn serious. Shakespeare used the form to convey tragic madness: Ophelia, when jilted by Hamlet, rhymes suggestively about the unfaithfulness of men. Sometime around 1569, Elizabeth I poured her feelings into “The Doubt of Future Foes,” a poem about her relationship with her cousin and enemy, Mary Queen of Scots, that used a limerick-like form.

But the experiment in gravity didn’t take, and before long limericks had found a permanent position in the world of silliness, immortalized in collections like “Mother Goose.” The limerick’s popularity with children made it the perfect material for Edward Lear (1812-1888). A tortured soul—among other things, he suffered from epilepsy, eye problems and unrequited love—he was often happiest entertaining the young. Though best known for such nonsense poems as “The Owl and the Pussy-cat,” Lear wrote so many limericks that he started the Victorian craze for them.

By the 1860s, however, some people had turned the limerick from simple child fare to pure adult entertainment. The satirical weekly Punch Magazine had to cancel its popular limerick competition due to the high volume of smutty entries. In 1902 the editors of the Princeton Tiger unwittingly opened a Pandora’s box of rude verse after publishing the first “Nantucket” limerick.

Although the 20th-century humorist Ogden Nash offered “A flea and a fly in a flue / Were imprisoned, so what could they do?” and other “clean” limericks, many contemporary writers preferred to play in the gutter. When not contemplating the stars, science-fiction writer Isaac Asimov was down in the dirt with such gems as “Cleopatra’s a cute little minx / With a sex life that’s loaded with kinks.”

But the limerick endures. As novelist Salman Rushdie demonstrated in a 2011 Twitter post, it remains the perfect form for a witty put-down: ‘The marriage of poor Kim Kardashian / Was krushed like a kar in a krashian. / Her Kris kried, “Not fair! Why kan’t I keep my share?” / But Kardashian fell klean outa fashian.”