on sb's behalf meaning

• [American idiom]
  [doing something] as someone's agent; [doing something] in place of someone; for the benefit of someone.
  • I'm writing in behalf of Mr. Smith, who has applied for a job with your company.
  • I'm calling on behalf of my client, who wishes to complain about your actions.
  • I'm calling in her behalf
  • I'm acting on your behalf
  

• behalf:    Noun: behalf  ...
• in behalf of:    [American slang]pr ...
• on behalf of:    [American idiom][d ...