In the seminal novel The Great Gatsby by F. Scott Fitzgerald, the titular character of Jay Gatsby is a showman and playboy whose ostentatious lifestyle draws the attention of the protagonist and narrator of the story, a young journalist called Nick Carraway, whose enquiries into the life and secrets of the great Gatsby is what ultimately propels the events of the story forth. Gatsby himself imitates the avuncular and amiable diction of an eccentric oldster of incalculable wealth, that he encounters early in the story. Wanting to emulate the mannerisms of the oldster, Gatsby becomes essentially a parody of the oldster. Talking further on about the critical role of the old man and how he impacts Gatsby life would spoil the overall twists in the story.
Nevertheless, Gatsby has a particular way of speaking, as mentioned above, and one word that marks his speech significantly is the appellation old sport.
Before we dive into how the Japanese translator, known by the online alias of Kareha (枯葉), has rendered this term, we need some history on what it actually is.
The term is akin to words like "buddy, friend, pal, mate, dude", that is markers of relationship between people. Old sport joins the ranks of dear fellow, old chum and old bean, archaic gentlemanly ways to refer to ones friends, where in the Great Gatsby it comes almost akin to a sentence ending.
This term serves two purposes: one is to show the reader and to that extent the characters of the story that Gatsby is a cultured and amiable gentleman, and the second that his emulation of the oldster has gone as far as to erode his original self, which becomes critical later in the story.
Originally meaning "a jest" (from the French derived disport) then "pastime" and in American English we see the emergence of "stylish man, dandy" hence its eventual usage as an affectionate term, Etymology Online suggests this usage is from as early as the 1860s thus the term has quite the legacy.
You can still hear an echo of old sport in the term the older expression "being a good sport", that is one that gracefully admits their defeat in a competitive activity such as sports, thus the term sport complete with the affectionate prefix old creates this old fashioned term of reference.
Fitzgerald would not only have been aware of its usage, but used it purposely to fashion the image of Gatsby as that of a member of the upper-class - a member of the New Money, rather than the Old Money, even if his mannerisms tend toward the latter.
Enter Sonkou (尊公):
Taking a particular line from when Gatsby uses the term for the first time, first Fitzgerald's original and then the Japanese translation by Kureha:
“Want to go with me, old sport? Just near the shore along the Sound.”
(Chapter 3)
「尊公、御一緒に如何でしょうか？ ほんの海峡沿いの海岸辺りまで」
"Sonkou, go-issho ni ikaga deshouka? hon no kaikyou zoi no kaiganatari made"
"Old sport, would you care to join me? It is merely along the channel and up towards the coast-line." (Kureha's translation)
So, the term sonkou that is used here to translate old sport literally means "exalted duke", but is an obscure honorific second person pronoun that also can be used politely to refer to another person's father, hence he is in actuality calling his interlocutors "revered pops" or some such, albeit more formal. Gatsby also uses polite terms such as go-issho ni, which is the honorific form of issho ni ("to accompany [someone]") as well as ikaga, the more courteous synonym of dou ("how"), thus reflecting his chivalrous if dated speech style in Japanese:
Returning to sonkou (尊公), the Japanese online dictionary repository, Kotobank, has the following to say on the term. My own notes are bracketed:
1. (noun) Respectful term for another person's father. Sonpu (尊父, "revered father"). (attested in) "Book of Jin - History of the Emperor Jianwen" (〔晉書‐簡文帝紀〕)
2. (substitution) Second person pronoun. Shows respect towards one's interlocutor. Mostly used by males towards other males. Sonku (尊君, lit. "revered lord," second person pronoun) (attested in) Vocabulario da Lingoa de Iapam (1603 - 1604) (日葡辞書（1603‐04）)
(From the entry listing definitions from the Seisenban Nihonkokugo Daijiten)
It then goes on to quote from a comedy from the early 1800s.
Which shows that this term is of even greater antiquity than old sport, but nevertheless shares its old fashioned nuances of gentlemanly formality which is also coupled with his polite speech patterns.