English.

1. Kay is a very old Celtic male and female name, derived from the Latin Caius/Gaius. Kay is now virtually confined to use as a female name.

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 is also a nick-name or pet name for many names beginning with the letter K-, particularly for Kathleen or Katherine/Katharine. This usage has increased the popularity of Kay as a given name.