The Collaborative International Dictionary of English v.0.48:

Tolerable \Tol"er*a*ble\, a. [L. tolerabilis: cf. F.
   tol['e]rable. See Tolerate.]
   1. Capable of being borne or endured; supportable, either
      physically or mentally.
      [1913 Webster]

            As may affect the earth with cold and heat
            Scarce tolerable.                     --Milton.
      [1913 Webster]

   2. Moderately good or agreeable; not contemptible; not very
      excellent or pleasing, but such as can be borne or
      received without disgust, resentment, or opposition;
      passable; as, a tolerable administration; a tolerable
      entertainment; a tolerable translation. --Dryden.
      [1913 Webster] -- Tol"er*a*ble*ness, n. --
      Tol"er*a*bly, adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tolerable
    adj 1: capable of being borne or endured; "the climate is at
           least tolerable" [ant: intolerable, unbearable,
           unendurable]
    2: about average; acceptable; "more than adequate as a
       secretary" [syn: adequate, passable, fair to middling,
       tolerable]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "tolerable":
   OK, acceptable, adequate, admissible, all right, allowable,
   average, bearable, better than nothing, betwixt and between,
   common, decent, dull, endurable, fair, fair to middling, fairish,
   good enough, goodish, indifferent, insipid, lackluster, livable,
   mediocre, medium, middling, moderate, modest, namby-pamby,
   not amiss, not bad, not half bad, not so bad, of a kind, of a sort,
   of sorts, okay, ordinary, passable, presentable, pretty good,
   respectable, run-of-the-mill, satisfactory, so so, so-so,
   sufferable, sufficient, supportable, sustainable, tedious, tidy,
   unexceptionable, unexceptional, unimpeachable, unobjectionable,
   vapid, wishy-washy, workmanlike