co-sign
verb /ˌkəʊ ˈsaɪn/
  /ˌkəʊ ˈsaɪn/
 [intransitive, transitive] (North American English)Verb Forms
| present simple I / you / we / they co-sign |    /ˌkəʊ ˈsaɪn/   /ˌkəʊ ˈsaɪn/  | 
| he / she / it co-signs |    /ˌkəʊ ˈsaɪnz/   /ˌkəʊ ˈsaɪnz/  | 
| past simple co-signed |    /ˌkəʊ ˈsaɪnd/   /ˌkəʊ ˈsaɪnd/  | 
| past participle co-signed |    /ˌkəʊ ˈsaɪnd/   /ˌkəʊ ˈsaɪnd/  | 
| -ing form co-signing |    /ˌkəʊ ˈsaɪnɪŋ/   /ˌkəʊ ˈsaɪnɪŋ/  | 
- to sign a legal document for somebody else, in addition to their own signature, in order to promise that an agreement will be kept
- See if your parents will co-sign.
 - co-sign for somebody Since you don't have a job, someone must co-sign for you.
 - co-sign something (for somebody) Her dad co-signed the lease for her apartment.