The Collaborative International Dictionary of English v.0.48:

Improbation \Im`pro*ba"tion\, n. [L. improbatio.]
   1. The act of disapproving; disapprobation.
      [1913 Webster]

   2. (Scots Law) The act by which falsehood and forgery are
      proved; an action brought for the purpose of having some
      instrument declared false or forged. --Bell. Improbative

Bouvier's Law Dictionary, Revised 6th Ed (1856):

IMPROBATION. The act by which perjury or falsehood is proved. Tech. Dict.
h.t.