The Collaborative International Dictionary of English v.0.48:
Unharness \Un*har"ness\, v. t. [1st pref. un- + harness.] 1. To strip of harness; to loose from harness or gear; as, to unharness horses or oxen. --Cowper. [1913 Webster] 2. To disarm; to divest of armor. --Holinshed. [1913 Webster]WordNet (r) 3.0 (2006):
unharness v 1: remove the harness from; "unharness a horse" [ant: harness, tackle]