The Collaborative International Dictionary of English v.0.48:

On \On\ ([o^]n), prep. [OE. on, an, o, a, AS. on, an; akin to D.
   aan, OS. & G. an, OHG. ana, Icel. [=a], Sw. [*a], Goth. ana,
   Russ. na, L. an-, in anhelare to pant, Gr. 'ana`, Zend ana.
   [root]195. Cf. A-, 1, Ana-, Anon.]
   The general signification of on is situation, motion, or
   condition with respect to contact or support beneath; as:
   [1913 Webster]

   1. At, or in contact with, the surface or upper part of a
      thing, and supported by it; placed or lying in contact
      with the surface; as, the book lies on the table, which
      stands on the floor of a house on an island.
      [1913 Webster]

            I stood on the bridge at midnight.    --Longfellow.
      [1913 Webster]

   2. To or against the surface of; -- used to indicate the
      motion of a thing as coming or falling to the surface of
      another; as, rain falls on the earth.
      [1913 Webster]

            Whosoever shall fall on this stone shall be broken.
                                                  --Matt. xxi.
                                                  44.
      [1913 Webster]

   3. Denoting performance or action by contact with the
      surface, upper part, or outside of anything; hence, by
      means of; with; as, to play on a violin or piano. Hence,
      figuratively, to work on one's feelings; to make an
      impression on the mind.
      [1913 Webster]

   4. At or near; adjacent to; -- indicating situation, place,
      or position; as, on the one hand, on the other hand; the
      fleet is on the American coast.
      [1913 Webster]

   5. In addition to; besides; -- indicating multiplication or
      succession in a series; as, heaps on heaps; mischief on
      mischief; loss on loss; thought on thought. --Shak.
      [1913 Webster]

   6. Indicating dependence or reliance; with confidence in; as,
      to depend on a person for assistance; to rely on; hence,
      indicating the ground or support of anything; as, he will
      promise on certain conditions; to bet on a horse; based on
      certain assumptions.
      [1913 Webster +PJC]

   7. At or in the time of; during; as, on Sunday we abstain
      from labor. See At (synonym).
      [1913 Webster]

   8. At the time of; -- often conveying some notion of cause or
      motive; as, on public occasions, the officers appear in
      full dress or uniform; the shop is closed on Sundays.
      Hence, in consequence of, or following; as, on the
      ratification of the treaty, the armies were disbanded;
      start on the count of three.
      [1913 Webster +PJC]

   9. Toward; for; -- indicating the object of some passion; as,
      have pity or compassion on him.
      [1913 Webster]

   10. At the peril of, or for the safety of. "Hence, on thy
       life." --Dryden.
       [1913 Webster]

   11. By virtue of; with the pledge of; -- denoting a pledge or
       engagement, and put before the thing pledged; as, he
       affirmed or promised on his word, or on his honor.
       [1913 Webster]

   12. To the account of; -- denoting imprecation or invocation,
       or coming to, falling, or resting upon; as, on us be all
       the blame; a curse on him.
       [1913 Webster]

             His blood be on us and on our children. --Matt.
                                                  xxvii. 25.
       [1913 Webster]

   13. In reference or relation to; as, on our part expect
       punctuality; a satire on society.
       [1913 Webster]

   14. Of. [Obs.] "Be not jealous on me." --Shak.
       [1913 Webster]

             Or have we eaten on the insane root
             That takes the reason prisoner?      --Shak.
       [1913 Webster]

   Note: Instances of this usage are common in our older
         writers, and are sometimes now heard in illiterate
         speech.
         [1913 Webster]

   15. Occupied with; in the performance of; as, only three
       officers are on duty; on a journey; on the job; on an
       assignment; on a case; on the alert.
       [1913 Webster +PJC]

   16. In the service of; connected with; a member of; as, he is
       on a newspaper; on a committee.
       [1913 Webster]

   Note: On and upon are in general interchangeable. In some
         applications upon is more euphonious, and is therefore
         to be preferred; but in most cases on is preferable.
         [1913 Webster]

   17. In reference to; about; concerning; as, to think on it;
       to meditate on it.
       [PJC]

   On a bowline. (Naut.) Same as Closehauled.

   On a wind, or On the wind (Naut.), sailing closehauled.


   On a sudden. See under Sudden.

   On board, On draught, On fire, etc. See under Board,
      Draught, Fire, etc.

   On it, On't, of it. [Obs. or Colloq.] --Shak.

   On shore, on land; to the shore.

   On the road, On the way, On the wing, etc. See under
      Road, Way, etc.

   On to, upon; on; to; -- sometimes written as one word,
      onto, and usually called a colloquialism; but it may be
      regarded in analogy with into.
      [1913 Webster]

            They have added the -en plural form on to an elder
            plural.                               --Earle.
      [1913 Webster]

            We see the strength of the new movement in the new
            class of ecclesiastics whom it forced on to the
            stage.                                --J. R. Green.
      [1913 Webster]

WordNet (r) 3.0 (2006):

on it
    adv 1: on that; "text and commentary thereon" [syn: thereon,
           on it, on that]