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.
from Wiktionary, licensed under Creative Commons - Attribution - Sharealike,
On Saturdays, barbers sat their customers in the shade on the porch of the Store, and troubadours on their ceaseless crawlings through the South leaned across its benches and sang their sad songs of The Brazos while they played juice harps and cigar-box guitars.
Maya Angelou. I Know Why the Caged Bird Sings (1969)
---
I ain' got nuffn but a coase comb en a piece o' paper, en a juice-harp; but I reck'n dey wouldn' take no stock in a juice-harp."
Mark Twain. The Adventures of Huckleberry Finn
---
[during The Jumping Jews of Jerusalem scene, a Jew's harp supplies the melody]