Source : Webster's Revised Unabridged Dictionary (1913)

Unmask \Un*mask"\, v. t. [1st pref. un- + mask.]
   To strip of a mask or disguise; to lay open; to expose.

Unmask \Un*mask"\, v. i.
   To put off a mask. --Shak.

Source : WordNet®

unmask
     v 1: reveal the true nature of; "The journal article unmasked the
          corrupt politician" [syn: {uncloak}]
     2: take the mask of; "unmask the imposter" [ant: {mask}]