The Collaborative International Dictionary of English v.0.48:

Penetrable \Pen"e*tra*ble\, a. [L. penetrabilus: cf. F.
   p['e]n['e]trable.]
   Capable of being penetrated, entered, or pierced. Used also
   figuratively.
   [1913 Webster]

         And pierce his only penetrable part.     --Dryden.
   [1913 Webster]

         I am not made of stones,
         But penetrable to your kind entreats.    --Shak.
   [1913 Webster] -- Pen"e*tra*ble*ness, n. --
   Pen"e*tra*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

penetrable
    adj 1: admitting of penetration or passage into or through; "a
           penetrable wall"; "penetrable defenses" [ant:
           impenetrable]
    2: capable of being penetrated; "penetrable defenses"

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "penetrable":
   accessible, apprehensible, approachable, articulate, assailable,
   attackable, attainable, available, beatable, cognizable,
   come-at-able, comprehensible, conquerable, easily understood,
   easy to understand, exoteric, expugnable, fathomable, findable,
   getatable, gettable, intelligible, knowable, obtainable, open,
   open to, openable, permeable, pervious, plumbable, porose, porous,
   pregnable, prehensible, procurable, reachable, readable, scrutable,
   securable, surmountable, to be had, understandable, vincible,
   vulnerable, weak, within reach