The Collaborative International Dictionary of English v.0.48:

Countersign \Coun`ter*sign"\ (-s?n`; 277), v. t. [imp. & p. p.
   Countersigned (-s?nd`); p. pr. & vb. n. Countersigning.]
   [Counter- + sign: cf. F. contresigner.]
   To sign on the opposite side of (an instrument or writing);
   hence, to sign in addition to the signature of a principal or
   superior, in order to attest the authenticity of a writing.
   [1913 Webster]