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]