It is forbidden to speak demeaningly of one's friend, even if it be absolute truth. And this is termed everywhere by Chazal "lashon hara." (For if there were in his words an admixture of falsehood, by which his friend is demeaned even more, this is in the category of "motzi shem ra" [spreading a false report], in which his sin is far greater). And the speaker [of lashon hara] transgresses a negative commandment, viz. (Vayikra 19:16): "Do not go talebearing among your people." And this [lashon hara] is also in the category of rechiluth.