WordNet: 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"