Fortunato keeps repeating the word "Amontillado" at the beginning of the story when Montresor first tells him he has purchased a whole cask of the gourmet Spanish wine. Poe's purpose in having Fortunato keep repeating that single word is apparently to show that the man is tremendously interested in it. It is a mistake to assume that Fortunato is mainly anxious to drink some Amontillado. It would not be that difficult for him to find Amontillado for sale in places that served liquor by the glass or sold it by the bottle. Besides that, Amontillado is a sipping sherry, an after-dinner drink, a cordial. Fortunato is already intoxicated on Italian wine that is normally drunk by the tumblerfuls. He is unlikely to want to mix sherry with what he has already been drinking. It might even make him sick. What interests him is the commercial value of Amontillado. He is rich. He could buy an entire cargo of the "pipes" of Amontillado and make a small fortune by bottling it and selling it off in smaller quantities--a few bottles at a time. A "pipe" of wine contains 126 gallons, according to the college dictionaries. He doesn't have to hurry about disposing of the wine, since it would only improve in quality and value as it aged in oaken casks. (Both Montresor and Fortunato refer to the cask as a pipe.) But Montresor warns Fortunato repeatedly that it may not be genuine Amontillado but only ordinary Spanish sherry. Fortunato has to taste it himself before he thinks about investing in a big quantity in wooden casks. That is why he is so strongly motivated to go to Montresor's vaults that very night. He is afraid of losing out on a bargain. Since he hasn't heard about the Amontillado arriving in port, he assumes it is still generally unknown--but that could change by tomorrow morning. He especially doesn't want Luchesi to hear about it, because Luchesi, unlike Montresor, probably could also afford to buy the entire shipload.
The entire point of the journey down into Montresor's vaults has been to taste the wine named Amontillado. Montresor lured him into the situation by saying he had bought something the was said to be Amontillado but he wasn't sure. Then Montresor had mentioned many times that Luchesi could tell if the wine was Amontillado, but Fortunato was too proud to turn around and kept going, At the end, Fortunado realizes that he has been tricked and the purpose of the journey was not to taste wine but to kill him. He is dumbfounded and keeps repeating the word as if saying the word will make the Amontillado appear and he will be safe. Fortunato is beginning to realize what a fool he has been and is hoping Montresor is just playing a joke on him. So he keeps saying, "But the Amontillado," hoping Montresor will say something like, Oh, I forgot, here is the Amontiado." and all will be back to normal. Then they can have a good laugh later when they remember the event in the future. Unfortunately, this is not the case and Fortunato realizes all dialogue is useless and he is silent.