Cosign (verb)

1. To sign a document along with one or more other people, typically as a witness or as a guarantee of the document's authenticity.
2. To sign a document jointly with someone else, typically as a witness or as a guarantee of the document's authenticity.

Origin:

From the latin word "con" meaning "together" and the word "sign" meaning "to write one"s name on a document".

Examples:

1. He cosigned the loan with her to help her improve her credit.
2. She cosigned the lease with him to help him get the apartment.
3. Cosigning a loan means that you are responsible for the debt if the borrower defaults.
4. You will be asked to cosign a loan if the borrower has poor credit.
5. Cosigning a lease means that you are legally responsible for any damage or unpaid rent.