The Collaborative International Dictionary of English v.0.48:

Passable \Pass"a*ble\, a. [Cf. F. passable.]
   1. Capable of being passed, traveled, navigated, traversed,
      penetrated, or the like; as, the roads are not passable;
      the stream is passablein boats.
      [1913 Webster]

            His body's a passable carcass if it be not hurt; it
            is a throughfare for steel.           --Shak.
      [1913 Webster]

   2. Capable of being freely circulated or disseminated;
      acceptable; generally receivable; current.
      [1913 Webster]

            With men as with false money -- one piece is more or
            less passable than another.           --L'Estrange.
      [1913 Webster]

            Could they have made this slander passable.
                                                  --Collier.
      [1913 Webster]

   3. Such as may be accepted or allowed to pass without serious
      objection; adequate; acceptable; tolerable; admissable;
      moderate; mediocre.
      [1913 Webster +PJC]

            My version will appear a passable beauty when the
            original muse is absent.              --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

passable
    adj 1: able to be passed or traversed or crossed; "the road is
           passable" [ant: impassable, unpassable]
    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:

62 Moby Thesaurus words for "passable":
   OK, acceptable, accessible, adequate, admissible, agreeable,
   all right, allowable, alright, attainable, average,
   better than nothing, betwixt and between, decent, dull, fair,
   fair to middling, fairish, good enough, goodish, indifferent,
   insipid, lackluster, mediocre, medium, middling, moderate, modest,
   namby-pamby, navigable, negotiable, not amiss, not bad,
   not half bad, not so bad, of a kind, of a sort, of sorts, okay,
   open, presentable, pretty good, reachable, respectable,
   satisfactory, so so, so-so, sufficient, tedious, tenable, tidy,
   tolerable, traversable, unblocked, unexceptionable, unexceptional,
   unobjectionable, unobstructed, vapid, viable, wishy-washy,
   workmanlike