In F. Scott Fitzgerald's "The Great Gatsby," Gatsby calls Nick "old sport" as a term of endearment. The phrase also references Gatsby's manufactured affectations and his transition from poor James Gatz to rich Jay Gatsby.
"Old sport" is Jay Gatsby's favorite term of endearment for his friends. Gatsby uses the term 41 separate times in F. Scott Fitzgerald's text. He calls not only Nick Carraway "old sport," but also Tom Buchanan and, it is implied, all of Gatsby's friends and acquaintances.
Gatsby uses the term "old sport" as both a term of endearment and a way to separate himself from other men. No other character in the novel uses the term — only Gatsby. It is one of his catchphrases, and it is a way to distinguish himself. It is also a deliberate affectation. This turn of phrase did not come from James Gatz's poor childhood; instead, Gatz adopted the phrase when he transitioned his identity into the wealthy, successful Jay Gatsby.
In the 2013 film adaptation of "The Great Gatsby," screenwriters Baz Luhrmann and Craig Pearce imply that Gatsby learned the expression "old sport" from his rich benefactor Dan Cody. However, this interpretation is not in F. Scott Fitzgerald's original novel.