Source : Webster's Revised Unabridged Dictionary (1913)

Thereon \There*on"\, adv. [AS. ??ron. See {There}, and {On}.]
   On that or this. --Chaucer.

         Then the king said, Hang him thereon.    --Esther vii.
                                                  9.

Source : WordNet®

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