The reader should not assume that the coat of arms Montresor describes is anything but Montresor's spontaneous invention. He hasn't been telling Fortunato the truth up to this point; why should he be telling him the truth now? Montresor has been acting in a zany fashion since he got his victim down the stairs. He has had a drink of wine and is vastly relieved that the worst part of his task is over. He pretends to be a Mason and shows Fortunato the trowel with which he intends to wall him up. Montresor may not even have a family coat of arms. Fortunato may suspect that and is just being disingenuous when he inquires. Most of the "thousand injuries" have probably been the spiteful jibes of a rich Italian insider reminding a poor French outsider of his inferior status. Montresor may be inventing the kind of coat of arms he would like to have--a huge foot crushing a snake. The Latin motto is probably an invention too, since it is so appropriate. Montresor probably knows that his intended victim is an ignoramus who doesn't even understand Latin and therefore can't sense that he is being subtly threatened with murder.
Poe's story, "The Cask of Amontillado" is full of foreshadowing and symbols. The coat of arms of Montresor, the aggressor in the story, is no exception.
When he mentions the coat of arms, the description is that of a golden foot crushing a snake whose fangs are embedded in the foot's heel. The motto, 'Nemo me impue lacessit' means "No one strikes me with impunity."
This motto and the images in the family's shield suggest that Fortunato is not going to be fortunate at all...rather, the opposite. Obviously, from the speeches Montresor delivers, he feels as though Fortunato has struck him with impunity. Fortunato, then, is the snake who has bitten Montresor's foot. Montresor intends to crush the snake one and for all, and by the end of the story, the reader and Fortunato realize he has done just that. By bricking him into a wall far away from where anyone will be able to hear his distress calls seals his fate (no pun intended) to become like the skeletons they encounter in the cellar.
Gold--the color of the foot--has long suggested fortune, wealth, good luck. Snakes have just as often symbolized an evil or forboding source...take the snake who beguiles Eve into eating the forbidden fruit, or the snakes in Harry Potter (symbol of the Slytherin House, the actual snake in one of the books), for that matter, as examples.