This is the 5338th most frequent Arabic word.
ثلثي
Two-thirds.
Here the word 'ثلثي' represents a fractional part of a object capacity.
ثلثي الكوب مملوء.
Two-thirds of the cup is full.
Here 'ثلثي' signifies a portion of the content consumed or completed.
قرأت ثلثي الكتاب.
I read two-thirds of the book.
In this example, 'ثلثي' quantifies a part of a group of people.