There are times when we are allowed to say something negative about someone else if we do not mention their names. This applies only if you are sure the person you are talking to will never figure out who you are talking about. It is still not a preferable or good thing to speak negatively about anyone whether or not you mention names. Negative speech powerfully affects the speakers and the listeners.
On Shabbat you tell your friend "Could you imagine? I saw someone in shul today talking straight through davening. Could you believe such a thing. He bothers to come to shul and doesn't even pray properly." Even though your friend most likely will never figure out who you spoke about, it is still not a commendable thing to do and more than it affects the person being spoken about, it affects you, the speaker.
(Purity of Speech by C.T. Friedman)
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