
Benjamin P. answered 09/07/19
Expert in English and Financial Proofreading
Like many words in the English language, "genius" has more than one definition. Dickens (a genius himself) uses the word in more than one sense. Early in the book he uses it in its original definition to mean "the attendant spirit of a person or place." The closest word I can think to compare it to is "guardian."
Later he definitely uses it in one of the senses we are most familiar with--genius for something, as in having a gift or talent for something. In chapter 15 it's very hard to tell. He appears to be using it sarcastically to mean the opposite of a high I.Q.