A coupling nut is typically a metal fastener which is threaded on the inside. It is designed to secure two parts together by screwing the coupling nut onto the outside of another part, often a threaded rod. The outside of a coupling nut is usually hex shaped so that a wrench can tighten or loosen it.n additional type of coupling nut contains something called a sight hole. This is a hole in the outside of the nut which travels all the way through the material.