Reprinted from the article “Leshon Ha-ra” in The Encyclopedia of the Jewish Religion, revised edition (1986), edited by R.J. Zwi Werblowsky and Geoffrey Wigoder, published by Adama Books.
Lashon hara [also known as leshon ha-ra or loshen horoh] [is] scandal-mongering. Lashon ha-ra is considered to be prohibited by the Bible on the basis of Leviticus 19:16, “You shall not go up and down as a slanderer [in some translations: talebearer] among your people,” and is frequently condemned in the Book of Proverbs.
The rabbis [of classical Judaism in late antiquity], in inveighing against it, often resorted to hyperbolic language, e.g. in saying that slander, talebearing, and evil talk were worse than the three cardinal sins of murder, immorality, and idolatry. Of one who indulges in lashon hara they say that he denies the existence of God, and that the Almighty declares “I and he cannot live in the same world” (Babylonian Talmud Arakhin 15b).
Rabbinic law distinguishes between various categories of talebearing (rekhilut), slandering, scandalmongering, etc. Every kind of trafficking in evil report or rumors—whether true or not—by carrying them from one person to another, or by relating unpleasant or harmful facts about another, is forbidden. The rabbis forbade even “the dust of lashon hara” [avak lashon hara], i.e., lashon hara by insinuation, as in saying “do not mention so-and-so for I do not wish to tell in what he was involved,” or in praising a person to his enemy since this also invites lashon hara.
Both the teller of and the listener to lashon hara are guilty of transgression, even if the person spoken about is present at the conversation. If a person publicizes unpleasant facts about himself, he who repeats them has not indulged in lashon hara.
The most thorough discussion of the halakhic and moral aspects of lashon hara is in Israel Meir Kagan’s Hafetz Hayyim.