a man's detachable insert (usually starched) to simulate the front of a shirt

A dickie (sometimes known as a tuxedo front or tux front) is a type of false shirt-front designed to be worn with a tuxedo, usually attached to the collar and then tucked into the cummerbund. The rigid plastic dickie came into fashion in the latter years of the 19th century, and was one of the first successful commercial applications of celluloid.