1. Sign jointly
   "Husband and wife cosigned the lease";
   - co-sign
    
2. Sign and endorse (another person's signature), as for a loan
   "Her parents cosigned her student loan application";
   - co-sign

Sounds like: cosine, co-sign

Derived forms: co-signing, co-signs, co-signed, cosigns, cosigning, cosigned

Type of: endorse, formalise [Brit], formalize, indorse [rare], validate

Encyclopedia: Cosign