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?
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
a plum in your mouthBRITISH
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.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012