Gell-Mann chose the whimsical name of "quarks" for these constituents. This word appears in the phrase "three quarks for Muster Mark" in James Joyce's novel, Finnegan's Wake .
The revolutionary part of their idea was that they had to assign the quarks electric charges of 2/3 and -1/3 in units of the proton charge: such charges had never been observed. At first the quarks were regarded as mathematical fiction, but experiments have convinced physicists that quarks do exist.