The Collaborative International Dictionary of English v.0.48:

Discreet \Dis*creet"\, a. [Compar. Discreeter; superl.
   Discreetest.] [F. discret, L. discretus separated (whence
   the meaning reserved, prudent), p. p. of discernere. See
   Discern, and cf. Discrete.]
   1. Possessed of discernment, especially in avoiding error or
      evil, and in the adaptation of means to ends; prudent;
      sagacious; judicious; not rash or heedless; cautious.
      [1913 Webster]

            It is the discreet man, not the witty, nor the
            learned, nor the brave, who guides the conversation,
            and gives measures to society.        --Addison.
      [1913 Webster]

            Satire 's my weapon, but I 'm too discreet
            To run amuck, and tilt at all I meet. --Pope.
      [1913 Webster]

            The sea is silent, the sea is discreet.
                                                  --Longfellow.
      [1913 Webster]

   2. Differing; distinct. [Obs.] --Spenser. -- Dis*creet"ly,
      adv. -- Dis*creet"ness, n. Discrepance

WordNet (r) 3.0 (2006):

discreetly
    adv 1: with discretion; prudently and with wise self-restraint;
           "I sent for the sergeant of the platoon both men were in
           and asked him to try to find out discreetly what lay
           behind this" [ant: indiscreetly]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "discreetly":
   artfully, astutely, cannily, carefully, cautiously, circumspectly,
   cleverly, considerately, cunningly, discerningly, easy, gingerly,
   guardedly, heedfully, intelligently, judiciously, knowingly,
   mindfully, pawkily, providently, prudently, rationally, reasonably,
   regardfully, sagaciously, sensibly, shrewdly, smartly,
   thoughtfully, understandingly, wisely, with care, with caution