The Collaborative International Dictionary of English v.0.48:

Opisthography \Op`is*thog"ra*phy\, n. [Gr. 'o`pisqen behind +
   -graphy.]
   A writing upon the back of anything, as upon the back of a
   leaf or sheet already written upon on one side. [R.]
   --Scudamore.
   [1913 Webster]