Adjective
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secret and sly or sordid
"backstairs gossip"
"his low backstairs cunning"- A.L.Guerard
"backstairs intimacies"
"furtive behavior"
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marked by quiet and caution and secrecy
taking pains to avoid being observed
"a furtive manner"
"a sneak attack"
"stealthy footsteps"
"a surreptitious glance at his watch"