Source : Webster's Revised Unabridged Dictionary (1913)

Thievish \Thiev"ish\, a.
   1. Given to stealing; addicted to theft; as, a thievish boy,
      a thievish magpie.

   2. Like a thief; acting by stealth; sly; secret.

            Time's thievish progress to eternity. --Shak.

   3. Partaking of the nature of theft; accomplished by
      stealing; dishonest; as, a thievish practice.

            Or with a base and biosterous sword enforce A
            thievish living on the common road.   --Shak.
      -- {Thiev"ish*ly}, adv. -- {Thiev"ish*ness}, n.

Source : WordNet®

thievishly
     adv : in a manner characteristic of a thief