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. (n.) The signature of a secretary or other officer to a writing signed by a principal or superior, to attest its authenticity.

3. (n.) A private signal, word, or phrase, which must be given in order to pass a sentry; a watchword.

ID card John Hancock OK X accept accredit affirm amen approve assure authenticate authorize autograph avouch badge calligram calling card card certify christcross cipher confirm cosign countermark countersignature counterstamp credentials cross device dog tag endorse endorsement give permission give the go-ahead give the imprimatur give thumbs up guarantee hand identification identification badge identification tag initial initials letter of introduction make a promise mark mark of signature monogram notarize open sesame pass pass on pass upon password permit pledge plight press card promise ratify rubber stamp sanction say amen to seal second secret grip serial number shibboleth sigil sign sign and seal sign manual signature signet subscribe to subscription support swear swear and affirm swear to tessera token troth undersign underwrite validate visa vise visiting card vouch vow warrant watchword word