under wraps - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Concealed or secret, as in The design for the new plant is under wraps. This idiom frequently is put as keep under wraps, meaning “keep secret,” as in Let's keep this theory under wraps until we've tested it sufficiently. It alludes to covering something completely by wrapping it up. [1930s]