Source : Webster's Revised Unabridged Dictionary (1913)

Surreptitious \Sur`rep*ti"tious\, a. [L. surreptitius, or
   subreptitius, fr. surripere, subripere, to snatch away, to
   withdraw privily; sub- under + rapere to snatch. See {Sub-},
   and {Ravish}.]
   Done or made by stealth, or without proper authority; made or
   introduced fraudulently; clandestine; stealthy; as, a
   surreptitious passage in an old manuscript; a surreptitious
   removal of goods. -- {Sur`rep*ti"tious*ly}, adv.

Source : WordNet®

surreptitious
     adj 1: marked by quiet and caution and secrecy; taking pains to
            avoid being observed; "a furtive manner"; "a lurking
            prowler"; "a sneak attack"; "stealthy footsteps"; "a
            surreptitious glance at his watch"; "someone skulking
            in the shadows" [syn: {furtive}, {lurking}, {skulking},
             {sneak(a)}, {sneaky}, {stealthy}]
     2: 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"
        [syn: {clandestine}, {cloak-and-dagger}, {hole-and-corner(a)},
         {hugger-mugger}, {hush-hush}, {on the quiet(p)}, {secret},
         {undercover}, {underground}]