a plum in your mouth

a plum in (one's) mouth

A manner of speech that is indicative of a high social class. Primarily heard in UK. Because he always sounds like he has a plum in his mouth, none of us working-class sods can stand to listen to him. Have you noticed that Sally's been speaking with a plum in her mouth ever since she started living in inner London?
See also: mouth, plum
Farlex Dictionary of Idioms.

a plum in your mouth

BRITISH
If someone speaks with a plum in their mouth, they speak with an accent that is typical of a high social class. While she does not talk with a plum in her mouth, she is very articulate. Note: This expression is used to show disapproval. Note: This is from the idea that the accent used by upper-class speakers of British English makes them sound as though they have a plum in their mouth.
See also: mouth, plum
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • a plum in (one's) mouth
  • plum
  • have a plum in (one's) mouth
  • have a plum in your mouth
  • speak with a plum in (one's) mouth
  • north and south
  • air of pretension
  • put words in (one's) mouth
  • put words in mouth
  • put words in/into somebody's mouth
References in periodicals archive
THE Kings Moss Farmers Market in St Helens will be hosting a fun competition to speak with a plum in your mouth.