I think I pieced it together:
One of the earliest and most famous people with this name seems to be Sir
Kay, one of the first Knights of the Round Table:
http://en.wikipedia.org/wiki/Sir_Kay
The Wikipedia articles says that he also was seneschal. So what's the work of a "seneschal"? "During medieval times the seneschal was the keeper of the keys in a noble house, ..."
I took this from here:
http://boilinghell.wikidot.com/the-seneschal
So, a famous
Kay was once the keeper of the keys of a "house",
King Arthur's castle. Does this make it the "meaning" of the name? No, if you ask me, but it's certainly enough that somebody states it as such, and from then on all the "baby names" websites copy it from each other...