(1):
(v. t.) 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.
(2):
(a.) The signature of a secretary or other officer to a writing signed by a principal or superior, to attest its authenticity.
(3):
(a.) A private signal, word, or phrase, which must be given in order to pass a sentry; a watchword.