ذِكْرُكَ أَخَاكَ بِمَا يَكْرَهُ
“Mentioning your brother in a way that he would dislike.”
Backbiting is not only a question of whether the words are true. Unnecessary disclosure of someone's fault or an unwelcome characteristic while they are absent can violate their dignity. The same ethical concern can arise in messages, written comments, insinuations or degrading imitations.
Example
Repeating a colleague's real weakness merely to entertain others illustrates backbiting. By contrast, a necessary and proportionate report to a responsible person to prevent harm or seek a legitimate right is a different situation with its own rules and limits.
A useful question is: Is there a genuine need to mention this, and am I saying more than that need requires?