Answer :
Because that's part of Gatsby's characterization, which is also only introduced in the third chapter.
We can arrive at this answer because:
- "Old sport" is a term used by Gatsby frequently.
- This term represents his inequity among millionaires.
- This shows that even though he is a millionaire, he doesn't fit in and is not looked upon favorably by this society.
Gatsby is only introduced in the third chapter of the story, and Fitzgerald does this to heighten the suspense and mystery about this character. Therefore, introducing the term "old sport" in the middle of the third paragraph is part of Gatsby's characterization and allows the reader to learn more about him.
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