In the folk tradition, I love folk etymology.
Harps were carried across Europe in the 16th Century by peddlars. Through the centuries itinerant peddlars have been Jews. Villagers buying them would have referred to them as Jew's harps. QED
But the question then is: in any other language than English where there is the similarity in sound to jaws and juice, is there any nomenclature for the instrument that reveals any ehnic or cultural or commercial source connection to Jews? Is the 'gew' of gewgaw just coincidentally only a letter away from Jew and could it have been pronounced Jewgaw originally? I just excavated my old Jew's harp from the bottom of its tomb, and still can't get more than one note and the percussion effect as the tang raps my front teeth.