Casmurro is the word that gives the book its name, and Bentinho explains its meaning in the very first chapter. He says the nickname was born on a train, when he dozed off while a young man from the neighborhood recited verses to him, and he warns the reader: "Do not consult dictionaries." According to him, the word does not have there the sense of stubborn or sullen that dictionaries usually give, but the one the people gave it: a silent man who keeps to himself.
The "Dom" came as irony, to attribute to him "airs of a nobleman," that is, the bearing of a gentleman. The nickname was given by the young man from the train and spread by the neighbors, who do not like the narrator's reclusive and silent habits. He says he was not offended, that even his friends in the city began to call him that, and that he found no better title for his narrative.
The detail matters because it shows the gesture that organizes the novel: the narrator fixes the meaning of words before the reader can examine them. The same will happen with Capitu's eyes and with the accusation against her.