The concept in halachah of Mesiach L’fi Tumo accords a casual remark made in conversation the status of testimony in beis din (rabbinical court). The classic case where this rule is applied is when a man goes overseas and does not return and someone casually mentions that he saw the man’s dead body. In certain specific situations, such remarks may be used to allow the missing man’s wife to remarry. The reasoning is that since the speaker apparently had no motive in mind when making the remark, we therefore assume that it is true.
However, in reference to accepting loshon hora, the Chofetz Chaim states that this halachic principle carries no weight. If in the course of conversation someone innocently mentions some negative information, we are not permitted to believe it. If the speaker mentions a situation in which someone is seen in an unfavorable light, we are required to seek a different understanding of what may have happened, thereby judging the person favorably. In general, whenever we glean negative information from someone’s innocent comments, we are required to disregard it.
The Talmud (Bava Metzia 58b) tells us that it is worse to insult someone than to hurt him financially. The Talmud explains: “This (hurtful words) affects his very self, whereas this (monetary wrongdoing) affects only his money”; “with this (monetary wrongdoing) restitution is possible, but with this (hurtful words), restitution is not possible.”
The halachah does not use the principle of Mesiach L’fi Tumo to award someone a monetary claim based on a casual remark. It follows, then, that using such comments as the grounds for insulting someone would be all the more forbidden.