The name Kaycee is a modern anglicized spelling of the ancient Irish Gaelic name Caitir. Caitir is derived from the Celtic words "cath" meaning battle and "rí" meaning king, together meaning "battle king" or "brave in battle". This name was popular among Irish warriors and nobility in the medieval period.

Caitir first appeared in the 8th century Irish epic poem Cath Ruis na Ríg ("The Battle of Ross na Ríg"). In this poem, the hero Caitir leads his clan's forces into battle against rival clans. The name is also mentioned in the ancient Book of Leinster, a 12th century manuscript containing many Celtic tales and histories.

The earliest known person named Caitir was an Irish king who ruled the kingdom of Conmaicne in modern-day County Leitrim around 750 AD. Another early bearer was Caitir mac Crimthainn, a 9th century prince from the Northern Uí Néill dynasty.

As the name spread across Europe through the Irish diaspora, it evolved into variations like Caitriona, Catharine, and eventually Katherine. One famous Katherine was the 14th century English princess Katherine Swynford, whose sons were legitimized and went on to found the Tudor dynasty.

In 16th century Scotland, the spelling Catriona was popular among Highland clans like the McGregors. The name belonged to Catriona McGrigor (c.1560-1633), celebrated for leading McGrigor clansmen in defending their lands.

By the 20th century, the spelling Kaycee emerged as an Americanized form, likely influenced by the Irish surname Casey. One notable bearer was Kaycee Moore (1924-1999), an award-winning American poet and writer.