Green’s Dictionary of Slang

(US black) a piano.

1939		P.E. Miller Down Beat’s Yearbook of Swing n.p.: eighty-eight (88) : piano.
1941		Pic (N.Y.) 24 June 24: What Do You Know About Music? [...] 18. What instruments are sometimes referred to as ‘woodpile,’ ‘plumbing,’ ‘licorice stick,’ ‘88,’ ‘skins’?
1953		L. Durst Jives of Dr. Hepcat (1989) 5: The kid that’s blowing the box is in stout shape letting his fingers idle boppingly over the ‘88’ the kiddy with the skins is sitting em up.
1967		J. Thompson ‘Hell’ in Fireworks (1988) 190: The world-you is making the ol’ eighty-eighty rise up [...] and declare itself, a weavin’ and a rockin’.
1977		J.L. Dillard Lex. Black Eng. 70: The piano itself has been the eighty-eight, the box, and any number of other things.
1995		B. Short (con. 1949) Saloon Singer 250: I am still amazed to discover how many actors can play a piano. Think of Kevin Kline and Michael Moriarty, and Diana Lynn, who had quite a way with the eighty-eights.
2002		R. Gordon Can’t Be Satisfied 108: Otis Spann [. . .] played the eighty-eights with fluidity, his left hand rumble as agile as his right hand tinkle.