The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

surreptitiously
    adv 1: in a surreptitious manner; "he was watching her
           surreptitiously as she waited in the hotel lobby" [syn:
           surreptitiously, sneakily]