The Montresor family crest (or "arms") is "A huge human foot d'or, in a field azure; the foot crushes a serpent rampant whose fangs are imbedded in the heel." That is (in plain English), a huge golden human foot on a sky-blue background; the foot is crushing a snake with the head in profile (the archaic definition of "rampant"), and the lizard's fangs are sunk into the heel.
The implication of this imagery is that Montresor's family may occasionally get "bitten," but they win in the end (by crushing their enemies).
Fortunato doesn't put this together, possibly because he is drunk. He says, "And the motto?"
Montresor replies: "Nemo me impune lacessit." In English, that is, "No one cuts me with impunity." (Or, in clear modern English, "No one hurts me and gets away with it").
Fortunato merely says: "Good!" He is drunk (as mentioned), but also completely oblivious to what Montresor has in mind. Considering the fact that Montresor is clearly a bit mad, it's entirely possible that Fortunato hasn't even done anything truly bad to him; if this is the case, he has not reason to believe Montresor plans to harm him.