Green’s Dictionary of Slang

1. (US black) a piano player.

1935		Ted Yates This Is New York 5 Apr. [synd. col.] There’s a Sugar Hill ‘hot’ spot that wants [...] a piano plunker who can ‘beat it out’.
1935		D. Burley Chicago Defender 14 Sept. 5: The piano plunkers, the horn tooters, the bass fiddlers and drum beaters.
1944		Ted Yates ‘I’ve Been Around’ 24 Mar. [synd. col.] Pat Flowers, the piano plunker.

2. (US black) a player of any stringed instrument.

1940		D. Burley N.Y. Amsterdam News 28 Dec. 16: Harry Goodman, the famed bass plunker.
1941		D. Burley N.Y. Amsterdam News 4 jan. 16: The Guitar Plunker may pop in port any day now.

3. see plunk n. (2)