कूप means a well, मण्डूकः means a frog. A well is the birthplace and an abode of a frog. A frog in a well moves about in a well, but does not get a chance to go any where outside the well. It has neither known nor seen any other place except its birthplace. As a result he thinks that well is the whole world. It does not get any chance to be familiar or get acquainted with the wide world outside the well. On the basis of this condition of the frog, this Nyaya became prevalent.