poison pen
/ˌpɔɪzn ˈpɛn/adjective
Designating, of, or relating to a letter or letters written anonymously and with malicious, libellous, or abusive intent.
noun
A writer of a poison-pen letter. Also: a pen dipped in poison (used allusively with reference to the virulent style of writing characteristic of a poison-pen letter).
Origin
Early 20th century; earliest use found in Evening Post (Frederick, Maryland). From poison + pen.