in an undertone

in an undertone

In a subtle, low-key, or hushed manner; quietly or understatedly. Usually but not always used in reference to speech or sound. I heard him state in an undertone that he wasn't surprised at the poor reception, given management's inadequate budget for the project, but I don't think anyone else heard him. In an undertone, the film upends the cliché of violence as a means of resolution that is so often the focus of other action films.
Farlex Dictionary of Idioms.

in an ˈundertone

,

in ˈundertones

in a quiet voice: ‘I must leave now,’ he said in an undertone. OPPOSITE: at the top of your voice
Farlex Partner Idioms Dictionary
See also:
  • stage whisper
  • high-key
  • nudge
  • a nod and a wink
  • a nudge and a wink
  • à la
  • low-key
  • all manner of
  • all manner of somebody/something
  • below (one's) breath
References in classic literature
"Why, Wakem is making himself particularly amiable to your cousin," said Stephen, in an undertone to Lucy; "is it pure magnanimity?
The Mill on the Floss
`Call the next witness.' And he added in an undertone to the Queen, `Really, my dear, YOU must cross-examine the next witness.
Alices Adventures In Wonderland
The latter, without making any resistance, said, in an undertone:
Five Weeks in a Balloon
They sat side by side and chattered to one another, with smothered laughter: now and then they glanced at Philip and one of them said something in an undertone; they both giggled, and Philip blushed awkwardly, feeling that they were making fun of him.
Of Human Bondage
It is obligatory for butchers to say Takbeer preferably loudly or at least in an undertone so that butcher might listen to his voice; otherwise, slaughtered chicken becomes unlawful/permissible (Haram) to eat.
Not only concerned authorities but every responsible citizen should play his role to propagate this issue and bind butchers to say Takbeer loudly at the time of slaughtering or at least in an undertone so that chicken remains lawful/permissible (Halal) to eat and it is better to buy chicken only when chicken is slaughtered before customer.
Un-Islamic way of slaughtering