Undersign (verb)

1. To sign (a document) at the bottom or beneath a signature already made.
2. To write one's signature at the bottom of a document or on a signature line.

Origin:

Late middle english: from under- + sign.

Examples:

1. She undersigned the document and passed it to the next person for signature.
2. He undersigned the contract and sealed the deal.
3. The document was undersigned by the president and the vice president.
4. The undersigned parties agree to the terms of this contract.
5. The undersigned witness attests that the signature is genuine.