Montresor has been lying to Fortunato consistently since they met on the street. There is no reason to believe that Montresor is telling the truth in the following exchange:
“I forget your arms.”
“A huge human foot d'or, in a field azure; the foot crushes a serpent rampant whose fangs are imbedded in the heel.”
“And the motto?”
“Nemo me impune lacessit.”
“Good!” he said.
More is going on here than is immediately apparent. It seems likely that Fortunato is being disingenuous. Montresor is not an aristocrat. He does not have a coat of arms or a family motto. He knows that Fortunato is only being his usual cruel self. The coat of arms that Montresor invents is totally bizarre. Imagine a huge human foot! And a golden one, at that! Would the foot be bare? Would we see all the toes? Wouldn't a knight be wearing something protective on the foot? Wouldn't an enemy just laugh at a knight who had a big golden foot on his shield? Isn't Montresor intentionally making the picture look ridiculous?
The motto, of course, goes with the coat of arms--but both may be pure inventions. Fortunato is drunk and doesn't really understand why his intended insult didn't have the intended effect of making Montresor feel humiliated. It would seem, too, that Fortunato does not understand Latin. This may be Montresor's way of retaliating for the intended insult: He satisfies himself that Fortunato may be rich but is not educated.
Naturally the motto is appropriate if Montresor invents it to suit his present purpose. The fact that it is so appropriate actually suggests that it is an invention. Montresor is not murdering Fortunato because it is mandatory in his family tradition to exact revenge for injuries; he is killing him because he hates him. It is a personal matter, not a part of a family code of honor. In fact, it can hardly be said that what Montresor is doing is "honorable."
Montresor's family motto is "Nemo me impune lacessit," or "No one insults me with impunity." This is very fitting because the small injuries that Fortunado has caused Montresor over the years has pushed him past his control. He wants to punish Fortunado to the fullest for his insults. He does this by conning him into going into his catacombs (after a cask of wine that doesn't exist). Once they get to the end, Fortuanado is quite drunk and is easily chained to the wall and walled into a small brick room. He punishes him without any penalty, since no one is any longer in the house--all the servants were out for the evening.