Source : Webster's Revised Unabridged Dictionary (1913)

Apposable \Ap*pos"a*ble\, a. (Anat.)
   Capable of being apposed, or applied one to another, as the
   thumb to the fingers of the hand.

Source : WordNet®

apposable
     adj : capable of being placed opposite to something; "the thumb is
           opposable to the forefinger" [syn: {opposable}] [ant: {unopposable}]