What is the meaning of "old sport"?

In the film "The Great Gatsby," Mr. Gatsby says "Sorry, old sport, I thought you knew"; what is the meaning of old sport?

Solution 1:

"Old sport" is just a friendly term of endearment used between equals, like buddy or the decidedly more modern dude. Using it today would likely be considered amusingly stuffy or upper-crust.

Solution 2:

http://www.etymonline.com/index.php?term=sport&allowed_in_frame=0 sport (n.)

Original sense preserved in phrases such as in sport "in jest" (mid-15c.). Meaning "game involving physical exercise" first recorded 1520s. Sense of "stylish man" is from 1861, American English, probably because they lived by gambling and betting on races. Meaning "good fellow" is attested from 1881 (as in be a sport, 1913). Sport as a familiar form of address to a man is from 1935, Australian English.