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]