Green’s Dictionary of Slang

1. (orig. US) a harmonica, a mouth organ.

1887		Scribner’s Mag. Oct. 481/1: She displayed a flimsy red silk handkerchief and a child’s harp .
1903		Ade ‘Why “Gondola” Was Put Away’ in In Babel 40: I’d walked from Loueyville over to Terry Hut with a nigger that played the mouth-harp.
1972		L. Bangs in Psychotic Reactions (1988) 104: I dropped the harp, which was such a limited palette for an experimental artist anyway.
1989		J. Morton Lowspeak.
2010		K. Richards Life 88: Cyril Davies was a hell of a harp player, one of the best.

2. (US) a vibraharp or vibraphone.

1938

Pic (N.Y.) Mar. 7: jibing the vibes or hitting the harps. — playing the vibraphone.