The Collaborative International Dictionary of English v.0.48:
Unhead \Un*head"\, v. t. [1st pref. un- + head.]
[1913 Webster]
1. To take out the head of; as, to unhead a cask.
[1913 Webster]
2. To decapitate; to behead. [Obs.] --T. Brown.
[1913 Webster]