rat on someone—(sl.) report smb.’s wrongful activities to the police, etc.:
- Kent is willing to do anything to get out even rat on his fellow convicts in order to get an early parole.
rat out on someone—(sl.)
1. desert or betray smb.; leave a person at a critical time:
- He would never rat out on his family cause underneath it all he loves them.
2. = rat on someone:
- He was sentenced to six years but only served eighteen months because he agreed to rat out on his drug suppliers.