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®

surreptitiously
     adv : in a surreptitious manner; "he was watching her
           surreptitiously as she waited in the hotel lobby" [syn:
            {sneakily}]