The following is an excerpt taken from the book The Manners of the Knowledge Seeker by Abu ‘Abdillah Muhammad Sa’id Raslan.
This is when you mention something about your brother in his absence that he wouldn’t want you to mention.
For example, it is when you speak about a physical shortcoming, such as his being blearyeyed, one-eyed, cross-eyed, bald, too tall, too short, etc.
It is also when you speak about a shortcoming in his lineage, such as saying that one’s father is a bedouin, an Indian, a sinner, low-class, etc.
It is also when you speak about his character, such as by saying that he has bad manners, is stingy, arrogant, etc.
It is also when you speak about his clothing, such as by saying that his coat is too long, his sleeves are too wide, he wears dirty clothes, etc.
The proof for all of this is that the Prophet (peace be upon him) was asked about backbiting, and he said: “It is when you mention something about your brother that he doesn’t like.” They asked: “What if he actually is as we say?” He (peace be upon him) replied: “If he is as you say, you have backbitten him. If he is not as you say, you have slandered him.”107
And know that anything that can even be understood to be belittlement counts as backbiting, whether this is in the form of a word, a wink, a point of the finger, or a written word, as the pen is one of your two tongues.
And the most disgusting form of backbiting is the backbiting of the fake religious people, such as when someone is mentioned, and they say ‘thank God for saving us from entering upon the ruler,’ or that they say ‘we seek refuge with Allah from having no haya’,’ or ‘may Allah spare us,’ as they combine between belittling this person and praising themselves. One of them might even say ‘This poor person has been tested with many sins. May Allah Forgive him and us,’ thereby hiding his true intent by making this supplication.
And know that the one who listens to backbiting is an accomplice to it, and he doesn’t disassociate himself from this crime until he speaks against it. If he fears for himself from speaking against it, he must at least hate it in his heart, and he should get up or change the subject if he can.
107 Reported by Muslim (2589), Abu Dawud (4874), and at-Tirmidhi (1934)