The Collaborative International Dictionary of English v.0.48:

Moon \Moon\, v. t. [imp. & p. p. Mooned; p. pr. & vb. n.
   Mooning.]
   1. To expose to the rays of the moon.
      [1913 Webster]

            If they have it to be exceeding white indeed, they
            seethe it yet once more, after it hath been thus
            sunned and mooned.                    --Holland.
      [1913 Webster]

   2. To expose one's naked buttocks to (a person); -- a vulgar
      sign of contempt or disrespect, sometimes done as a prank.
      [PJC]