1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): *under wraps
Fig. concealed; suppressed. (*Typically: be ~; have ~; hold ~; keep ~.) • We kept the candidate’s conviction under wraps until after the election. • The plan we had under wraps had to be scrapped anyway.
McGrawhill's American Idioms And Phrasal Verbs