Chafetz Chaim, Part One, The Prohibition Against Lashon Hara, Principle 3
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וּרְאֵה עוֹד אֶת גֹּדֶל אִסוּר לָשׁוֹן הָרָע, (ב) שֶׁאֲפִלּוּ אֵינוֹ מְדַבֵּר מִתּוֹךְ הַשִּׂנְאָה וְלֹא נִתְכַּוֵּן בְּהַסִפּוּר לְגַנּוֹתוֹ, רַק אֲמָרוֹ דֶּרֶךְ שְׂחוֹק וְדֶרֶךְ קַלּוּת רֹאשׁ, אַף עַל פִּי כֵן כֵּיוָן שֶׁעַל פִּי אֱמֶת דִּבְרֵי גְּנַאי הוּא, אָסוּר מִן הַתּוֹרָה.
Chofetz Chaim, Silverstein translation
And see further how great the issur of lashon hara is. For even if he does not speak out of hatred and does not intend in what he says to demean him, but speaks only in jest and from light-headedness, still, since in truth these are demeaning words, it is forbidden by the Torah.