"surreptitious" in the adjective sense
1. furtive, sneak, sneaky, stealthy, surreptitious
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"
2. clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, secret, surreptitious, undercover, underground
conducted with or marked by hidden aims or methods
"clandestine intelligence operations"
"cloak-and-dagger activities behind enemy lines"
"hole-and-corner intrigue"
"secret missions"
"a secret agent"
"secret sales of arms"
"surreptitious mobilization of troops"
"an undercover investigation"
"underground resistance"
Source: WordNet® (An amazing lexical database of English)
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