Ghibah (backbiting) is a major and grave sin. Unfortunately, many Muslims have taken this matter lightly and have set their tongues free to backbite others and spread rumors. Pointing out the heinousness of this crime Almighty Allah says: “Neither shall you backbite one another. Would any of you like to eat the flesh of his dead brother…” (al-Hujurat 49:12). A Muslim should always be on guard not to engage in such sinful conversations and should rather wisely advise those who engage in it to hold their tongues and spare themselves the punishment of the Hereafter.
On the question of whether saying something bad about unidentified person is considered backbiting or not, Dr. `Abdul-Fattah Idrees , a professor of comparative jurisprudence said:
Mentioning an unidentified person who is not known to the listeners neither in name nor in attributes is not prohibited. Talking about persons who are ambiguous to the listeners do not fall within the scope of ghibah.