Source : Webster's Revised Unabridged Dictionary (1913)

Outspan \Out*span"\, v. t. & i. [D. uitspannen.]
   To unyoke or disengage, as oxen from a wagon. [S. Africa]

Source : WordNet®

outspan
     v : remove the yoke or harness from; "unspan the draft animals"
         [ant: {inspan}]
     [also: {outspanning}, {outspanned}]