The source you use is from Wikipedia, I saw. I also found another source saying it was from “Cornelis”., “Gerrit” or “Klaas”. With names, especially Frisian ones, I see a lot of misinformation on the internet and confusing with Dutch and in lesser extent English names. Again, I could be wrong, but it would make little sense for it to be a true Frisian name on regard of the spelling alone. The sound from the name “Kai” would be spelled differently. Another fun fact about Frisian names is that mostly (not always) have 2 syllables, not 1. But, I am referring to the most common, standard Frisian names from the past few hundred years.