Definition of "plonky"

They both possess a light, slightly nasal voice, but whereas Siffre has his excellent guitar work to back up his vocals, O'Sullivan only has that same old plonky piano and his arranger.

1973, Beat Instrumental & International Recording, numbers 122-127, page 59

Clement Ishmael's plodding arrangements for lone, plonky piano, and the unlovely voices of two-thirds of the cast, do not help: even a sublimely sexy song like You Can Leave Your Hat On is rendered flat and toneless.

1996, Theatre Record, volume 16, numbers 9-15, page 871