The Collaborative International Dictionary of English v.0.48:
Unlash \Un*lash"\, v. t. [1st un- + lash.] (Naut.) To loose, as that which is lashed or tied down. [1913 Webster]WordNet (r) 3.0 (2006):
unlash v 1: untie the lashing of; "unlash the horse" [ant: lash]