(archaic) An intimate friend; a person who is treated as a cousin but is not a blood relation.
Etymology: 1500s. The most popular explanation is that 'cater-' means "crooked" or "angled". Similar to cater-corner (diagonally across from).
In my part of the country, "cater-corner" has been corrupted to "kitty corner"; as far as I know, cater-cousin never experienced such changes.