Sarcasm is an ironic or satirical remark tempered by humor. Mainly, people use it to say the opposite of what's true to make someone look or feel foolish. For example, let's say you see someone struggling to open a door and you ask them, "Do you want help?" If they reply by saying, "No thanks. I'm really enjoying the challenge," you'll know they're being sarcastic. Sarcasm is all about the context and tone of voice, which is why it works better verbally. It's something you'll know when you hear it.
"Fortunato" is an Italian derivation of the Roman proper name "Fortunatus." It refers to a Latin adjective which means "blest" or "fortunate." It is known popularly referenced in the Bible in 1 Corinthians 16:17, in which Fortunatus is one of the Seventy Disciples and serves as an ambassador to the Corinthian church. St. Paul writes in this verse:
I was glad when Stephanas, Fortunatus, and Achaicus arrived, because they have supplied what was lacking from you.
"Fortunatus," thus, went on to become relatively popular in the Catholic tradition, with many saints, martyrs, and clergymen taking up the name. This--as the other educators have pointed out--is deeply ironic given Fortunato's indulgent behavior throughout the story. Fortunato does not appear to possess the graces and qualities of a man of faith; rather, he seems to gratify his every whim and desire, no matter how base or low--drinking, gossiping, cavorting, and partying his way through life. The way in which he dies--being paved behind a wall while drunk--is hardly beatific or holy. He does not perish as a martyr, but rather as a fool.
"Will you walk into my parlor?" Said the spider.
