English.

1. Kay is a very old Celtic male and female name, derived from the Latin Caius/Gaius. Its use as a male name has been out of favour for a long time.

The Spanish male name Cayo comes from the same source as does the Old Welsh name Kei. The latter is thought to mean "fiery" or "rejoicer".

2. Kay may also be a pet name for any forename beginning with the letter K-. This usage has increased the popularity of Kay as a given name.