The name Kelsy is derived from the Gaelic word "cailleach," which means "old woman" or "nun." Its roots can be traced back to ancient Celtic folklore and mythology, where the cailleach was often depicted as a powerful, wise, and sometimes fearsome figure associated with winter and the transition between seasons.

In early Irish and Scottish traditions, the cailleach was revered as a goddess or a mythical being who embodied the harsh yet regenerative forces of nature. She was believed to wield control over the weather, especially during the winter months, and her appearance was said to signal the onset of spring. The name Kelsy likely emerged as a way to honor or invoke the protective and nurturing aspects of this mythological figure.

One of the earliest recorded instances of the name Kelsy can be found in the Irish Annals of the Four Masters, a 17th-century chronicle that mentions a woman named Cailleach Dé Danann in connection with ancient Irish legends. Other early references to variations of the name, such as Cailleach Bhéarra and Cailleach Bheara, appear in medieval Scottish and Irish texts.

Throughout history, several notable individuals have borne the name Kelsy. In the 12th century, Cailleach of Beare was a legendary Irish pirate queen who terrorized the waters off the coast of County Cork. Another famous bearer of the name was Kelsy Grammer, an American actor and producer best known for his role as Frasier Crane in the television sitcoms "Cheers" and "Frasier" (born 1955).

In literature, the name Kelsy appears in the works of Scottish author George MacDonald, who featured a character named Caillen or Caillionta in his 19th-century fantasy novels. Additionally, Kelsy Michauda was a prominent American artist and illustrator in the early 20th century, known for her work in children's books and magazines (1884-1966).

During the Middle Ages, Kelsy Figueres was a Spanish noblewoman and courtier who served as a lady-in-waiting to Queen Isabella I of Castile (c. 1470-1540). More recently, Kelsy Mayer was an influential American lawyer and civil rights activist who played a pivotal role in the desegregation of public schools in the 1950s and 1960s (1905-1986).