"voice intermediary between the soprano and the tenor, lowest female voice," 1730, contralt, from Italian contralto, from contra, from Latin contra "against, opposite" (see contra) + alto (see alto). As "person with a contralto voice," 1776; as an adjective, 1769.
In medieval music, in which the melody was either in a middle voice or passed from one voice to another, and which utilized only male singers, the upper voice was naturally called altus. As music for mixed voices developed, that female voice which was nearest the altus, and thus most contrasted with it, was called contr' alto. [Century Dictionary]