Green’s Dictionary of Slang

(orig. US black) a piano.

1939		P.E. Miller Down Beat’s Yearbook of Swing n.p.: mothbox : a piano.
1988		(con. 1930s) D. Jenkins Fast Copy 22: Up in Harlem, where canaries sang while hepcats played the mothbox, hides, plumbing, and licorice sticks, they had listened to the rhythms of Fletcher Henderson.
2000		(con. 1940s–60s) Décharné Straight from the Fridge Dad 132: Mothbox Piano.