plum + -y. In the sense of a voice, because of the supposed similarity to speaking with a plum in one's mouth.
plummy (comparative plummier, superlative plummiest)
- Of, pertaining to, containing, or characteristic of plums
- (informal) desirable; profitable; advantageous
- For the sake of getting something plummy. — G. Eliot.
- (of a voice) rich, mellow and carefully articulated, especially with an upper-class accent
2014 March 31, Roger Cohen, “The case for Scotland”, The New York Times:
- The fact that David Cameron, the conservative prime minister, is a plummy-voiced, Eton-educated, upper-class Brit from central casting has played into [Alex] Salmond's hands.