RichThesaurus
behind someone's back
vs
on the Q.T.
behind someone's back
prep
Without somebody's knowledge; secretly.
on the Q.T.
prep
Quietly; in a secretive manner; clandestinely.
How often have the words
behind someone's back
and
on the Q.T.
occurred in a corpus of books?
(source:
Google Ngram Viewer
)