The Collaborative International Dictionary of English v.0.48:

Pretty \Pret"ty\, adv.
   In some degree; moderately; considerably; rather; almost; --
   less emphatic than very; as, I am pretty sure of the fact;
   pretty cold weather.
   [1913 Webster]

         Pretty plainly professes himself a sincere Christian.
                                                  --Atterbury.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Pretty \Pret"ty\, a. [Compar. Prettier; superl. Prettiest.]
   [OE. prati, AS. pr[ae]ttig, pr[ae]tig, crafty, sly, akin to
   pr[ae]t, pr[ae]tt, deceit, trickery, Icel. prettugr tricky,
   prettr a trick; probably fr. Latin, perhaps through Celtic;
   cf. W. praith act, deed, practice, LL. practica execution,
   practice, plot. See Practice.]
   1. Pleasing by delicacy or grace; attracting, but not
      striking or impressing; of a pleasing and attractive form
      a color; having slight or diminutive beauty; neat or
      elegant without elevation or grandeur; pleasingly, but not
      grandly, conceived or expressed; as, a pretty face; a
      pretty flower; a pretty poem.
      [1913 Webster]

            This is the prettiest lowborn lass that ever
            Ran on the greensward.                --Shak.
      [1913 Webster]

   2. Moderately large; considerable; as, he had saved a pretty
      fortune. "Wavering a pretty while." --Evelyn.
      [1913 Webster]

   3. Affectedly nice; foppish; -- used in an ill sense.
      [1913 Webster]

            The pretty gentleman is the most complaisant in the
            world.                                --Spectator.
      [1913 Webster]

   4. Mean; despicable; contemptible; -- used ironically; as, a
      pretty trick; a pretty fellow.
      [1913 Webster]

   5. Stout; strong and brave; intrepid; valiant. [Scot.]
      [1913 Webster]

            [He] observed they were pretty men, meaning not
            handsome.                             --Sir W.
                                                  Scott.
      [1913 Webster]

   Syn: Elegant; neat; fine. See Handsome.
        [1913 Webster]

WordNet (r) 3.0 (2006):

pretty
    adv 1: to a moderately sufficient extent or degree; "pretty
           big"; "pretty bad"; "jolly decent of him"; "the shoes are
           priced reasonably"; "he is fairly clever with computers"
           [syn: reasonably, moderately, pretty, jolly,
           somewhat, fairly, middling, passably] [ant:
           immoderately, unreasonably]
    adj 1: pleasing by delicacy or grace; not imposing; "pretty
           girl"; "pretty song"; "pretty room"
    2: (used ironically) unexpectedly bad; "a pretty mess"; "a
       pretty kettle of fish"

Moby Thesaurus II by Grady Ward, 1.0:

99 Moby Thesaurus words for "pretty":
   a bit, a little, acceptably, adequately, adroit, aesthetic,
   aesthetically appealing, almighty, appealing, attractive, awfully,
   beauteous, beautiful, bonny, catchy, charming, clever, comely,
   cunning, cute, darling, decently, die, ducky, dulcet, elegant,
   endowed with beauty, euphonious, exceedingly, exquisite, extremely,
   eye-filling, fair, fairishly, fairly, fairly well, fetching, fine,
   flowerlike, good, good-looking, graceful, gracile, handsome,
   harmonious, heavy, in a measure, in a way, in some measure,
   incredibly, just, kind of, lovely, lyrical, mellifluous, melodic,
   melodious, mightily, mighty, moderately, more or less, musical,
   only too, passably, plaything, powerful, powerfully, presentably,
   pretty much, pretty well, pulchritudinous, quite, rather, real,
   really, reasonably, respectably, right, satisfactorily, scarcely,
   slightly, so, some, something, somewhat, sort of, terribly,
   terrifically, to a degree, to some extent, tolerably, toy, tuneful,
   unexceptionably, very, well enough, wicked, winsome, workmanlike