(כא) מָ֣וֶת וְ֭חַיִּים בְּיַד־לָשׁ֑וֹן...
(21) Death and life are in the power of the tongue...
You shall not go up and down as a talebearer (rachil) among your people; neither shall you stand idly by the blood of your neighbor: I am Adonai.
(א) לֹ֥א תִשָּׂ֖א שֵׁ֣מַע שָׁ֑וְא אַל־תָּ֤שֶׁת יָֽדְךָ֙ עִם־רָשָׁ֔ע לִהְיֹ֖ת עֵ֥ד חָמָֽס׃ (ס)
(1) You must not carry false rumors; you shall not join hands with the guilty to act as a malicious witness:
(ג) אָמְרוּ חֲכָמִים שָׁלֹשׁ עֲבֵרוֹת נִפְרָעִין מִן הָאָדָם בָּעוֹלָם הַזֶּה וְאֵין לוֹ חֵלֶק לָעוֹלָם הַבָּא. עֲבוֹדַת כּוֹכָבִים וְגִלּוּי עֲרָיוֹת וּשְׁפִיכוּת דָּמִים. וְלָשׁוֹן הָרַע כְּנֶגֶד כֻּלָּם. ... וְעוֹד אָמְרוּ חֲכָמִים שְׁלֹשָׁה לָשׁוֹן הָרַע הוֹרֶגֶת. הָאוֹמְרוֹ. וְהַמְקַבְּלוֹ. וְזֶה שֶׁאוֹמֵר עָלָיו. וְהַמְקַבְּלוֹ יוֹתֵר מִן הָאוֹמְרוֹ:
(3) Our Sages said: "There are three sins for which retribution is exacted from a person in this world and, [for which] he is [nonetheless,] denied a portion in the world to come: idol worship, forbidden sexual relations, and murder. Lashon horah is equivalent to all of them."...
In addition, they said: "Lashon horah kills three [people], the one who speaks it, the one who listens to it, and the one about whom it is spoken. The one who listens to it [suffers] more than the one who speaks it.”
EVIL TALK (LASHON HARA)
The most common, and perhaps the most serious speech problem is lashon hara, literally, "evil talk." It refers to any statement that is derogatory or potentially harmful to others -- even if it is true. Although there are other distinctions in Jewish law, the term lashon hara is also popularly used to include tale bearing (rechilut) and slander (motzei shem ra) or spreading lies.
HURTFUL WORDS (ONA'AT DEVARIM)
Words can cause more pain than any weapon. The Torah says that the greatest pain in this world is embarrassment. One who embarrasses another so the person blushes is judged as if he spilled blood.
DECEPTIVE TALK (GANEVAT DA'AT)
Fooling people with words is also problematic. Asking a salesman "How much is this item?" is deceptive if one has no intention of actually buying the item. In many other ways we mislead others through our speech, including flattery and boasting.
DISGUSTING SPEECH (NIVUL PEH)
Included in this category are such things as curse words, off-color jokes, or negative innuendo. What's wrong with saying an occasional curse word? The Torah teaches that the way one acts on the outside affects who one is on the inside. So even if a person is basically good, once he begins to speak in a crude way, his character will become negatively affected.
-- https://www.aish.com/jl/i/s/kosher_speech.html