In one theory, it means nyogo or koi who was a mother of Prince and Princess, but it is generally accepted that it was the name of ladies at the inner palace who were loved by the Emperor, such as nyogo and koi except for serving ladies.
皇子・皇女を生んだ女御、更衣を指すという説もあるが、一般的には天皇に寵愛を受けた女御、更衣など女官を除いた後宮の女性を指す呼称として用いられた。