lips are sealed, one's
One will reveal nothing, especially about a secret. For example, You can trust me with the details of the lawsuit—my lips are sealed. [Early 1900s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.