"Ignoramus" is a Latin word meaning an ignorant person. Fortunato probably does not really consider Luchesi an ignoramus but wants to create a bad impression of Luchesi in Montresor's mind because he is afraid Montresor will ask the other man to sample the Amontillado. Montresor keeps suggesting that he will consult Luchesi in order to make Fortunato anxious to sample it on that very night.

The third paragraph of Poe's story suggests that all three men--Montresor, Fortunato, and Luchesi--deal in luxury goods such as paintings, sculptures, antiques, jewelry, and gourmet wines. Fortunato is not motivated to drink Amontillado or to show off his connoisseurship or to accommodate his friend Montresor: he thinks he can make a lot of money if there is a whole shipload of Amontillado just arrived in port and selling it at bargain prices. He is afraid that Luchesi might buy the entire cargo if he finds out about it. But Fortunato has to sample the wine first in order to make sure it is the real Amontillado sherry. Montresor has told him three times, "I have my doubts."

Montresor has bought one cask of the Amontillado (he says). If he were sure it was genuine he would probably buy more as an investment. Amontillado in a wooden cask will last for decades and only improve with age. No doubt Fortunato is already planning to taste the wine, tell Montresor it is only inferior sherry, and then buy up the cargo himself. He pointedly does not ask Montresor where he got the wine because he doesn't want to show any interest in buying any of it himself. But this is his main motivation for rushing to taste the Amontillado that very night. If the wine is genuine, it would be easy enough for Fortunato to find the ship that brought it in from Spain without even questioning Montresor about it. There is a small fortune at stake here.