1963, from French tiers monde, formulated 1952 by French economic historian Alfred Sauvy (1898-1990) on model of the third estate (French tiers état) of Revolutionary France; his first world (The West) and second world (the Soviet bloc) never caught on.
thinkable
thinker
thinner
third
third degree
Third World
thirst
thirsty
thirteen
thirteenth
thirties