poison pen letter
noun /ˌpɔɪzn ˈpen letə(r)/
/ˌpɔɪzn ˈpen letər/
- an unpleasant letter that is not signed and is intended to upset the person who receives it
- He had been sending poison pen letters to people at work.
- He had been sending poison pen letters to his neighbours, hoping to make them move away.