In "The Cask of Amontillado" by Edgar Allan Poe, the narrator lies to Fortunato repeatedly from the time he encounters him on the street until the time he shackles him to the wall. There is no reason to believe that he is telling the truth when Fortunato asks about his coat of arms and he describes it as
"A huge human foot d'or, in a field azure; the foot crushes a serpent rampant whose fangs are imbedded in the heel."
The coat of arms seems too appropriate. It also seems terribly bizarre. Montresor is feeling jubilant now that he has succeeded in solving his greatest problem, which was to lure Fortunato down into his vaults without being recognized by anyone. Montresor has a rapier concealed under his roquelaire, while his victim is unarmed and grossly inebriated. Montresor can kill him any time he wants to, so he is acting in a zany fashion, partly induced by the wine he has consumed. He claims to be a Mason and shows Fortunato the trowel with which he intends to wall him up.
It may be that Montresor doesn't even have a coat of arms. Fortunato may be adding another injury to the thousand he has already inflicted when he says:
"I forget your arms."
If they have supposedly been friends for a long time, Fortunato ought to know more about Montresor's family. (This strongly suggests that they are both business associates but not really personal friends. Both deal in luxury goods such as oil paintings, antiques, gemmary, and old gourmet wines. Venice is a decaying city where aristocratic families are sometimes forced to sell off possessions in order to survive.) This is the very first time Fortunato has been to Montresor's palazzo, which is significant. Montresor may never have been invited to Fortunato's palazzo, or may have only been there on one or two occasions. Fortunato looks down on Montresor, a Frenchman, a relative johnny-come-lately to Italy. When he inquires about a coat of arms he may be thinking of forcing Montresor to confess that his family is bourgeois and never possessed one.
When Montresor describes his flamboyant and probably imaginary coat of arms, Fortunato, still hoping to catch him in a falsehood, asks:
"And the motto?"
Montresor may be inventing a motto to fit his imaginary coat of arms when he replies:
"Nemo me impune lacessit."
There is a good possibility that Fortunato does not even understand Latin and an equally good possibility that Montresor knows he doesn't. Fortunato merely says, "Good!" This is especially ironic, since Montresor is warning him he is in grave danger.
Whenever this famous story appears in an anthology, the editor usually takes pains to offer an English translation of the motto; but Poe (characteristically) does the reader the courtesy of assuming he or she can understand the Latin without help. We all know that "nemo" means "nobody" and that "me" must mean "me." The Latin "impune" is obviously close to the English "impunity," and "lacessit" suggests English words like "lacerate." So it isn't hard even for a non-Latinist to figure out that the motto is saying something like "Nobody injures me and gets away with it."
Fortunato's questions about the coat of arms and family motto suggest that he is in the habit of injuring Montresor by reminding him in various subtle ways of the differences in their family backgrounds, social status, and material possessions; while Montresor's deliberate lies suggest that he is looking forward to his final revenge.