RichThesaurus
off-the-books
vs
on the sly
off-the-books
on the sly
adv
Slyly, in an inconspicuous manner, so as not to be seen; secretly; stealthily.
How often have the words
off-the-books
and
on the sly
occurred in a corpus of books?
(source:
Google Ngram Viewer
)