Fortunado makes a hand gesture that is a secret signal of the Brotherhood of Masons.
Montresor seems to think that Fortunado has wronged him, so he brings him deep into the catacombs. Fortunado makes a gesture at one point, while he is drunk, and then asks Montresor if he is one of the Masons.
He laughed and threw the bottle upwards with a gesticulation I did not understand.
Fortunado then asks for further proof. Since Montresor is pretending to be a Mason, he wants him to make a hand signal as well.
“You? Impossible! A mason?”
“A mason,” I replied.
“A sign,” he said, “a sign.”
Since Montresor does not know the sign, did not recognize it, and can’t make it, he brings out his trowel and said this is a sign. Fortunado thinks it is funny, a jest that is a play on the word “mason.” He is too drunk to wonder why his friend has a trowel in the first place. Of course, Montresor plans to use it to brick his friend up, burying him alive in the catacombs.
Poe had one huge plotting problem in creating "The Cask of Amontillado." Montresor wants to lead Fortunato from the central part of Venice to his palazzo and down into his catacombs, from thence along winding tunnels to the spot where he plans to chain him to the granite wall and seal him in a niche with a stone wall. This journey would obviously take some time. Poe had to fill up that time with some action and some dialogue. It would seem natural for Fortunato to ask a lot of questions about this cask of gourmet sherry--yet he never once asks Montresor a question such as, "How much did you pay for it?" or "Who did you buy it from?" Poe had to invent distractions or else have Fortunato follow Montresor in perfect silence to his doom.
Poe provides Fortunato with a cold and a bad cough. This provides a partial explanation of the fact that Fortunato does not do too much talking. There is an interlude during which the two men get involved in a discussion of the Masons and another during which Montresor describes his coat of arms and his family motto. They stop to drink wine a couple of times while they are underground. Obviously Fortunato cannot ask questions while he is guzzling wine. Montresor distracts him on a couple of occasions by calling his attention to the nitre on the catacomb walls. But there is no mention of this cask of delicious Amontillado. Why?
Fortunato is supposed to be an expert judge of Amontillado. Montresor does not really have any Amontillado, so he would prefer not to have to answer a lot of questions about it, especially since Fortunato is an expert judge. Montresor may know nothing about this Spanish wine except the name. Edgar Allan Poe himself might have known little or nothing about Amontillado and preferred to avoid the subject himself. But something had to be said by these two men while they were walking to Montresor's palazzo, climbing down the stairs into the wine cellar, then winding their way through the catacombs to where the fictitious cask of wine was supposedly stored.
Two of the questions Fortunato might have asked are:
1. Why did you put the Amontillado so far away from the foot of the staircase?
2. Why didn't you simply tap the cask and bring a bottle of this Amontillado upstairs where we could sample it at ease in your living room?
The only conceivable explanation for Fortunato's strange lack of questions about the wine is that he does not want to show too much interest in it. He would like Montresor to think he is just doing him a favor, but actually he intends to buy some or all of the remaining cargo of Amontillado for himself. He assumes that Montresor is in a big hurry to have a connoisseur judge the authenticity of his wine because he intends to buy more of it at a bargain price before word gets out that it is available. (Presumably it is being sold off at a bargain price because everybody in Venice is drinking and celebrating, so nobody except Montresor knows the ship has arrived in port.) Montresor is a poor man and could only buy a few casks, but Fortunato is a rich man and could buy up an entire shipload of Amontillado--if he were sure it was genuine. He does not have to ask where Montresor purchased his cask. It would have to be aboard a newly arrived Spanish ship, and such a ship would be extremely easy to find in the harbor. Fortunato could go aboard and deal directly with the captain or purser. He probably is secretly planning to tell Montresor that his cask of wine is not genuine Amontillado, thereby eliminating him as a competitor. In other words, Fortunato expects to say the wine is not true Amontillado regardless of whether it is genuine or whether it is ordinary sherry.
Fortunato's "hand movement" and the talk about the Masons is just one of Poe's devices for filling up space and avoiding having to talk about the Amontillado.