One of the mysteries of the English language finally explained.
Designating, of, or relating to a letter or letters written anonymously and with malicious, libellous, or abusive intent.
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).
Early 20th century; earliest use found in Evening Post (Frederick, Maryland). From poison + pen.
poison pen/ˌpɔɪzn ˈpɛn/
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