It is a fraction. ) or at least unnecessary (I suppose it is a remnant of the slash in '2/3' in the mind of the writer) Better would be 'about two thirds of the population'. As an adjective, it should hyphenated-- 'a two-thirds majority of the Senate is required', 'a three-quarter length dress'; it is rare in this usage, though.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
It doesn't work for X'two-thirds population'-- with fractions less than 1, we usually use 'of a + singular noun' (vide Swan), hence 'thirds' remains a noun.