First, it is Luchesi, with only one c.
Luchesi is a very minor character who never even appears in this short story. The basic plot involves the narrator (Montresor) luring his enemy (Fortunato) into the catacombs beneath his palace so he chain him to the wall and bury him alive. The way he lures him is to talk about a cask of Amontillado (very fine sherry wine) that he has received. Montresor tells the drunk Fortunato that he is not sure whether the drink is in fact Amontillado. Montresor says he plans to ask Luchesi, another wine expert, to test it. This makes Fortunato want to be the one to test it and so follow Montresor into the catacombs. Whenever Montresor thinks Fortunato might be having second thoughts (because the cold in the catacombs is worsening his cough, for example) Montresor mentions that he can go to Luchesi. Of course, this only encourages Fortunato.
An ignoramus is someone who is very ignorant or unknowledgeable. Just as Fortunato reaches the last room in the catacombs, and when Montresor mentions Luchesi again, Fortunato says Luchesi is an ignoramus. Specifically, he means that Luchesi doesn't know anything about wines.
In "The Cask of Amontillado" the main character and narrator, Montressor, is trying to persuade his enemy, Fortunado, to come into his vaults to test a bottle of Amontillado for its validity. In order to do this he has to try to make it seem as though he has another person who would be just as good of a wine tester as Fortunado -- that person in Luchesi. Therefore, Montressor continues to bring up Luchesi by saying that he will just get Luchesi to test out the bottle of Amontillado instead.
In the story, Fortunado, speaking of Luchesi, states, "He is an ignoramous." By this, Fortunado means that Luchesi does not have any knowledge of wine and that he would not be of any help to Montressor ... he is basically saying that Luchesi is stupid and ignorant on the subject of wine.
"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.