(v. t.) To write one's name at the foot or end of, as a letter or any legal instrument.

OK accept accredit affirm amen approve assure attest authenticate authorize autograph back be sponsor for bond certify confirm cosign countersecure countersign endorse ensure give permission give the go-ahead give the imprimatur give thumbs up guarantee guaranty initial insure notarize pass pass on pass upon permit ratify rubber stamp sanction say amen to seal second secure sign sign and seal sign for sponsor stand behind stand up for subscribe to support swear and affirm swear to underwrite validate visa vise warrant