intransitive verb.
To speak spitefully or slanderously about (another).
intransitive verb.
To speak spitefully or slanderously about a person.
The Century Dictionary and Cyclopedia
To injure morally in a manner comparable to biting from behind; attack the character or reputation of secretly; censure, slander, or speak evil of in absence: rarely with a thing as object.
To slander or speak evil of the absent.
the GNU version of the Collaborative International Dictionary of English
transitive verb.
To wound by clandestine detraction; to censure meanly or spitefully (an absent person); to slander or speak evil of (one absent).
intransitive verb.
To censure or revile the absent.