The Collaborative International Dictionary of English v.0.48:

Unscrew \Un*screw"\, v. t. [1st pref. un- + screw.]
   To draw the screws from; to loose from screws; to loosen or
   withdraw (anything, as a screw) by turning it.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unscrew
    v 1: loosen something by unscrewing it; "unscrew the outlet
         plate"
    2: loosen by turning; "unscrew the bottle cap" [ant: screw]