Fortunato appears to be acting disingenuously when he asks Montresor about his family coat of arms and motto. He regards Montresor as a member of a lower class then himself and probably has asked many such disingenuous questions during their relationship with the sole intention of hurting Montresor's feelings. He apparently believes that Montresor has no coat of arms or motto. Montresor has been lying to Fortunato consistently since they meet on the street, so there is no reason to think he is telling the truth when he describes his coat of arms.
A huge human foot in gold—such a design on a shield would seem bizarre, surrealistic, or comical. Poe probably wanted the reader to realize that Montresor was only kidding Fortunato. The motto, Nemo me impune lacessit, meaning "Nobody injures me with impunity," is, of course, appropriate, but it seems almost too appropriate. Montresor must be taking advantage of the fact that Fortunato is drunk to make the man look like a fool. Montresor is telling him what his coat of arms and motto would be if he had a coat of arms and a motto. Fortunato is not only drunk but probably does not even understand Latin, so it is especially ironic that he answers, "Good!" when Montresor is virtually predicting what is soon going to happen to him.
Montresor himself has been doing a little drinking along with Fortunato, so his rather odd behavior, including showing Fortunato his trowel and claiming to be a Mason, is understandable. Also Montresor has been under pressure to get Fortunato off the street and down below; and now he is experiencing relief because he knows he has his victim in his power. Montresor has a rapier concealed under his cloak. He can kill the unarmed Fortunato any time he wants—although it would be preferable to lead him to the site when he intends to wall him up.