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The colloquial British-English noun fug-soccer, also fug-socker, designates indoor football.

This noun, which originated in public-school ¹ and university ² slang, is composed of:
– the noun fug, which designates a thick, close, stuffy atmosphere, especially that of a room overcrowded and with little or no ventilation;
– the noun soccer, also socker, which designates the game of football as played under the rules of the Football Association.

(Both the nouns fug and soccer, too, originated in British-English public-school and university slang.)

¹ Here, public school designates a private independent fee-paying secondary school—cf., below, quotation 3 (about Rugby School) and quotation 7 (about Harrow School).
² Here, the Universities of Oxford and Cambridge—cf., below, quotations 1, 2, 4, 5 & 6.

These are, in chronological order, the earliest occurrences of the noun fug-soccer, also fug-socker, that I have found:

1-: From Notes of the Day, published in The Oxford Review: The Undergraduates’ Journal (Oxford, Oxfordshire, England) of Saturday 30^th January 1897 [page 3, column 1]:

We are pleased to hear that the wet weather is again provided against for those who would take their exercise regularly, and prefer it in the dry. Of course there is always the Gymnasium, but that lacks the excitement of a game. Last term in objectionable weather men had either to turn out or slack, as the Racquet Courts in Museum-road were closed, and in consequence football—or to be exact—Fug Socker—was impossible. This term, although the premises just mentioned remain closed, the courts in Manor-road have been thrown open to the game, and people are thus enabled to prevent the slacking fiend from  enveloping them. Although at present only open from one till four, we think it would be a good idea to encroach half an hour each way, so as to  give as many as possible the opportunity of a  game, seeing that only a few can occupy a court at a time.

2-: From The “House” on Sport. By Members of the London Stock Exchange (London: Gale & Polden, Ltd., 1898), compiled and edited by W. A. Morgan [Rackets, page 245]:

The Universities have not done their duty by Rackets. It is only quite lately that Cambridge has built a court fit to play in. But at Oxford, we are ashamed to say, the case is even worse. A few years ago, it is true, a couple of courts existed, but so bad were they that every decent racket-player who went up was forced, after two or three trials, to give up the game in despair as far as Oxford was concerned. So the courts were given over to “Fug-soccer”—a kind of four-a-side football with a miniature ball and goals, in its way an excellent game, but hardly what racket-courts are built for.

3-: From The Meteor. Edited by Members of Rugby School (Rugby, Warwickshire, England) of Saturday 11^th June 1898 [page 54, column 2]:

The state of the Old Court is a serious danger to the standard of the School Racquets, and requires first attention. […] Hardly anyone is enthusiastic enough to face the manifold drawbacks of the Old Court in its present condition, and for all the use it is to the School, we might soon have seen it devoted, like those at Oxford, to the pastime known to the initiated by the name of “fug-soccer.”

4-: From The Episode of Vera and the Overcoat, a short story set at Oxford, by the English author Inglis Allen (1879-1943), published in The Idler (London, England) of February 1900 to July 1900 [page 427]:
—This story was reprinted in A Varsity Man: Passages in the Career of an Impressionable Undergraduate (London: C. Arthur Pearson, 1901):

The Youth took a stroll in the High, and bought some ties. Inside Shepperd’s he met Cardrake, of Trinity, another Old Cliffburian.
“Heard about old Brummles?” queried Cardrake.
“No—what?”
“Crocked.”
“No—really?”
“Knee,” said Cardrake, laconically. “Water, I believe. Six weeks’ touch. Fug soccer, of all things.”
“Harder,” said the Youth.

5-: From Oxford from Within (Philadelphia: George W. Jacobs & Co., [1910]), by the English author and journalist Hugh de Sélincourt (1878-1951) [chapter 2, § 8, page 70]:

[He] went on across the centre of the sloping green expanse, and out by the Nunnery along Church Road between the grounds of New College, Merton, and Balliol, past the old Racquet Courts once famous for “Fug Soccer,” now the home of the Oxfordshire Light Infantry.

6-: From Varsity Notes, by ‘Camox’, published in The Tatler (London, England) of Wednesday 8^th November 1911 [page 56, column 1]:

One is apt to wonder if someone will generously provide a racquet court for Oxford. It seems absurd that if a man  wants to play he has at present to go to town for a game. The  number of accomplished public-school  players who come up is very great, but the  lack of practice facilities naturally detracts from their game and more than half of them give up the game altogether. There are  plenty of sites, and  only the donor or donors wanted. The old courts where the fast and furious game of “fug soccer” used to be played, are, we believe, now a drill hall. To  renovate them would probably be as costly as to build new ones under the circumstances, but the sum required would not be appallingly large. Perhaps the Rhodes bequest may be elastic enough for the purpose, if not we suggest that the proctorial fines be used.

7-: From an account of Founder’s Day at Harrow, held on Thursday 7^th October 1915, published in The Mail (London, England) of Friday 8^th October 1915 [page 4, column 4]:

There were three or four house football games watched by one solitary mother and by a balloon that hovered over Harrow in the appropriately murky air. There were games of fives and squash going on, and a game of “fug soccer” was being played by some small boys in the schoolyard, which was so cheerful and vigorous as to be distinctly reviving to the spirits.