Cosigner

Hi there guys again! I have a question. I have looked up every English dictionary for this word, but I haven't found it in any of them. Does anybody know what it means? Does this word really exist in English?

Thanks in advance.

Both cosigner and co-signer are used widely. So are co-signing

WordReference's definitions are general:

one of two or more signers of the same document (as a treaty or declaration)
a signer in addition to the principal signer (to verify the authenticity of the principal signature or to provide surety)

http://www.wordreference.com/definition/cosigner

Merriam-Webster's definition is more specific to finances, which I believe it its most common use:

cosignatory; especially : a joint signer of a promissory note

http://www.m-w.com/dictionary/cosigner

If you want to take out a loan from a bank or other lending institution, but your credit rating is not sufficiently strong, you get a "co-signer" to "co-sign" the loan.

What that means is that if you fail to meet the terms of the loan repayment, then the co-signer will then be liable for the outstanding portion of the loan.