behind closed doors
In secret, privately. For example, The nominating committee always meets behind closed doors, lest its deliberations become known prematurely. Also see behind the scenes.
The Mystery Behind April’s NameWhile the first day of the fourth month of the year is sure to bring plenty of shenanigans, what isn't so certain is where the month's name originated.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.