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]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "mooning":
   Walter Mitty, absence of mind, absent, absentminded,
   absentmindedness, absorbed, absorption, abstracted, abstractedness,
   abstraction, bemused, bemusement, brown study, castle-building,
   daydream, daydreamer, daydreaming, daydreamy, depth of thought,
   dream, dreaming, dreamy, drowsing, ecstatic, elsewhere, engrossed,
   engrossment, fantasy, fantasying, faraway, fit of abstraction,
   half-awake, in a reverie, in the clouds, lost, lost in thought,
   meditative, moonraking, muse, museful, musefulness, musing,
   muted ecstasy, napping, nodding, oblivious, pensive, pipe dream,
   pipe-dreaming, preoccupation, preoccupied, rapt, reverie,
   somewhere else, stargazing, study, taken up, trance, transported,
   unconscious, woolgathering, wrapped in thought