1963, from Fr. tiers monde, formulated 1952 by A. Sauvy on model of the third estate (Fr. tiers état) of Revolutionary France; his first world (The West) and second world (the Soviet bloc) never caught on.
Underdeveloped or developing countries, as in The conditions in our poorest rural areas resemble those in the third world. This expression originated in the mid-1900s, at first denoting those countries in Asia and Africa that were not aligned with either the Communist bloc nations or the non-Communist Western nations. Because they were for the most part poor and underdeveloped, the term was transferred to all countries with those characteristics, and later still to poorer groups within a larger prevailing culture.