Likely a modern spelling of Casey, from an Irish surname meaning watchful or alert.
Cayse is a phonetic respelling of Casey, a name with deep Irish roots. The original Irish form, Cathasach, is an ancient Gaelic adjective meaning 'vigilant,' 'watchful,' or 'wakeful in battle' — qualities prized in warriors and leaders. The name belonged to several early Irish chieftains and saints, and as English-speakers anglicized Irish names through the centuries, Cathasach became Casey.
It entered the American consciousness most powerfully through folklore: the legendary engineer John Luther Jones, immortalized in the 1900 ballad as 'Casey Jones,' became a folk hero who held the whistle open as he died trying to save his passengers. Casey also entered American slang as a synonym for 'having everything in order.' For much of the twentieth century, Casey functioned primarily as a masculine name in the United States, buoyed by the 1949 poem 'Casey at the Bat' by Ernest Lawrence Thayer, in which a swaggering ballplayer strikes out in the pivotal moment — a portrait of American hubris that lodged itself in the culture permanently.
By the 1980s and 1990s, Casey had crossed over decisively to feminine use as well, becoming one of the earlier unisex names to achieve true gender balance. Television characters, from Casey Jones in the Teenage Mutant Ninja Turtles franchise to Casey Hughes on the soap opera As the World Turns, kept the name in circulation. Cayse, with its distinctive spelling, represents the personalization trend that accelerated in the 1990s and 2000s — the impulse to make a familiar sound uniquely one's own through orthographic variation.
The 'y' replaces the 'e' and the final 'e' is dropped, producing a form that looks simultaneously older (almost medieval) and more modern. For families who choose it, Cayse carries all of Casey's Irish vitality and American vernacular warmth in a form that feels a little more singular.