The word harp is often applied to a wide variety of instruments in English, including many (such as this one) that are not "harps" in the most specific sense. The origin of Jew's is uncertain, it is widely accepted that the instrument has no musicological or etymological connection with Jews and the word was capitalized due to the mistaken belief of a connection. Some have connected it to the Cleveland dialectal gew-gaw, related to the Old Norsegiga, compare modern Swedishgiga, Danishgige, GermanGeige(“fiddle”).[1] Others have suggested a connection with jaws,[2] the synonym jaw harp may have therefore come first. Other theories point to Frenchjeu-trompe(literally “toy-trumpet”),[3] compare Jew's-trump.
A musical instrument consisting of a flexible metal or bamboo “tongue” attached to a frame. This tongue is placed in the performer's mouth and plucked with the finger to produce a note of constant pitch. Melodies can be created by changing the shape of the mouth and causing different overtones.
1850, T. S. Arthur, “Deacon Smith and his Violin”, in Sketches of Life and Character[1], Philadelphia: J. W. Bradley, →OCLC, page 70:
The lad, his son, had obtained a Jew's-harp, and learned to play upon it the profane airs of "Yankee Doodle," "Hail Columbia," "St. Patrick's Day," and "Auld Lang Syne."