the.com/poison pen

means An anonymous or spitefully written letter designed to hurt, slander, or threaten someone, often while hiding behind politeness or correct grammar.

from The phrase surfaces in English in the early 20th century, naming a very specific nuisance: anonymous letters mailed to wound their recipient. 'Pen' stands in for writing itself, as it long has (the pen versus the sword), while 'poison' does the real work, casting words as a toxin slipped into an otherwise ordinary envelope. The term got a boost from notorious real-life 'poison pen' scandals — small towns thrown into turmoil by a flood of malicious unsigned letters — which made newspapers fond of the label, and fond enough that it stuck as the everyday name for venomous, often cowardly correspondence.