The Great Gatsby, old sport business

Could you please tell me the meaning of "business" in the following excerpt from The Great Gatsby?

“I’ve got it.” Gatsby examined the parted string, muttered “Hum!” in an interested way, and tossed the book on a chair.

“That’s a great expression of yours, isn’t it?” said Tom sharply.

“What is?”

“All this ‘old sport’ business. Where’d you pick that up?”

Thank you