Source : Webster's Revised Unabridged Dictionary (1913)

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.