The Collaborative International Dictionary of English v.0.48:

Thievish \Thiev"ish\, a.
   1. Given to stealing; addicted to theft; as, a thievish boy,
      a thievish magpie.
      [1913 Webster]

   2. Like a thief; acting by stealth; sly; secret.
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            Time's thievish progress to eternity. --Shak.
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   3. Partaking of the nature of theft; accomplished by
      stealing; dishonest; as, a thievish practice.
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            Or with a base and biosterous sword enforce
            A thievish living on the common road. --Shak.
      [1913 Webster] -- Thiev"ish*ly, adv. --
      Thiev"ish*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thievishly
    adv 1: in a manner characteristic of a thief