What is "poison-pen letters"

Usage examples of "poison-pen letters".

If poison-pen letters can get to you, you don't deserve any woman's love, let alone Rhoda's.

Wanting to see his wife, when poison-pen letters from home were accusing her of adultery?

Not that either accused the other of writing the poison-pen letters.

The phony charges, the trumped-up smears and lies and vicious poison-pen letters, mailed to Unity without signature.

In a normal culture they are the type of people who write poison-pen letters.