The British-English expression gyppy tummy (also gippy tummy, gyppie tummy, Egyptian tummy) denotes diarrhoea suffered by travellers, especially in Egypt.
—Synonyms: Pharaoh’s Revenge and Montezuma’s Revenge.

Note: The word gyppy is from:
– –gyp– in Egyptian;
– the suffix –y, used to form familiar diminutives.

The expression gyppy tummy occurs, for example, in the following by Jane Shilling, published in the London Evening Standard (London, England) of Wednesday 22^nd July 2009:

WILDEST DREAMS
7.30pm, BBC1
Reality TV goes on safari with this peculiar show. Nine “ordinary people”, as presenter Nick Knowles loftily calls them, are flown to Africa and told to film some wildlife. At the end of the process, someone will win a job at the Beeb’s Natural History Unit. Knowles keeps stressing how dangerous it is but the main danger for viewers is boredom-induced narcolepsy. The contestants seem a bland lot, apart from the inevitable lady with a gyppy tummy, and wildlife filmmaker James Honeyborne looks miserable in his role as judge.

The earliest occurrences of the expression gyppy tummy and variants that I have found are as follows, in chronological order—several of the texts indicate that this expression originated in British-Army slang:

1-: From Surgical work at the Military Hospital, Cairo, by J. C. Jefferson and O. H. Blacklay, of the Royal Army Medical Corps, published in The British Medical Journal of Saturday 12^th June 1915:

Abdominal surgery has been practically limited to appendicitis, of which we have dealt with 20 cases. The acute cases mostly occurred soon after our arrival in Egypt, when the heat was great, and there was an epidemic of diarrhoea among the troops. Four cases were accompanied by general peritonitis of extreme gravity, and owing to delay in diagnosis due to the prevalence of diarrhoea and abdominal pain, a condition known locally as “gippy tummy,” they did not come into our hands until a week after the onset of the disease.

2-: From A consulting surgeon in the Near East (London: Christophers, 1920), by Alfred Herbert Tubby (1862-1930)—V.A.D. is the abbreviation of Voluntary Aid Detachment, denoting a British organisation of civilians which provided nursing care for military personnel:

On account of the vast amount of work in Egypt a number of ladies belonging to the V.A. Detachment arrived there; many became seriously ill, and some died shortly after arriving, owing chiefly to intestinal complaints. Egypt is a country which requires getting used to, and nearly every one suffers at first from a form of exhausting subacute gastro-enteritis, colloquially known as “Gippy Tummy.” As in all subtropical regions, there are special dangers, such as the risk of dysentery, typhoid, etc.

3-: From A Journey to Egypt, published in the Staffordshire Sentinel (Stoke-on-Trent, Staffordshire, England) of Thursday 30^th December 1920—the reporter is on board a ship sailing from Italy to Egypt:

The passengers are a cosmopolitan crowd—English; French, Russians, Rumanians, Greeks, Egyptians, even Germans, whose very voices irritate the British, and people of no or every nationality. The British, as usual, cling together, and one gathers up odd stores of information—many and various remedies being recommended for a complaint openly discussed and commonly known as “Egyptian tummy”—to which it seems all newcomers must expect, sooner or later, to fall a victim.

4-: From Endemicity of intestinal protozoa, by H. M. Woodcock, of the Lister Institute of Preventive Medicine, published in The British Medical Journal of Saturday 5^th March 1921:

There is a well-known transient complaint, which was alluded to by our men as “gippy tummy,” characterized by sickness and diarrhoea, to which many are liable on first arriving in Egypt, however careful they may be as regards what is eaten and drunk.

5-: From Life on the Desert’s Fringe, published in The Gentlewoman and Modern Life (London, England) of Saturday 23^rd January 1926:

The Egyptian food takes a little getting used to. It is very unsafe to eat lettuce or any uncooked vegetable that may have been watered with Canal water, in which lurks the much-feared microbe, known familiarly as “Bill ’Arris.” It is also very unwise to drink fresh milk, although one gets very tired of the tinned variety. Even with care and circumspection, it is very difficult for new comers to avoid an attack of a peculiar illness, called “Gyppy Tummy.” Very devastating while it lasts, but mercifully it does not usually last long.

6-: From A Lover in Cairo, a novel by the British author and journalist George Valentine Williams (1883-1946), published in the North Mail Newcastle Daily Chronicle (Newcastle-upon-Tyne, Northumberland, England) of Thursday 28^th January 1926:

Simmons was in bed, Simmons whose proudest boast had ever been that she had never “known a day’s illness in her life”—qualified presumably in respect of mal de mer.
That ironclad spirit which had never struck its flag to man born of woman had lowered its colours to that insidious traitor known to travellers in the East as Gyppie tummy. What generations of hotel-keepers had failed to achieve—French rapacity, Swiss insolence, German bounce, Italian brigandage, she had bearded them all in her time—the fell visitant had accomplished. Gyppie tummy had “got the better” of Yours respectfully, H. Simmons.

7-: From The Thumb of Fat’ma, a short story set in Egypt, by George Valentine Williams, published in several U.S. newspapers in August 1927—for example in The Daily Argus-Leader (Sioux Falls, South Dakota, USA) of Saturday 6^th August 1927:

With characteristic perverseness Hobbsy fell ill of that unpleasant ailment indelicately named “Gyppie tummy,” leaving an outwardly sympathetic but inwardly somewhat indignant Betty to her own devices.

8-: From Egypt’s Morning Milk, an unsigned short story published in the Newcastle Morning Herald and Miners’ Advocate (Newcastle, New South Wales, Australia) of Saturday 14^th July 1928:

“If you want to catch typhoid, bubonic, smallpox, enteric, colic, jaundice, ‘gippy tummy,’ or plague, Sulliman’s goats will give you any or all of them,” said my neighbour.

9-: From a letter dated Cairo, Sunday 14^th December 1930, published in Hitting the high spots: Letters written during sixteen months of wandering over strange lands and among strange people (San Francisco: Privately printed for the author by Taylor & Taylor, 1931), by Claire K. Jones:

One day we stopped at the American University, sending in our cards to Dr. Watson, who received us immediately. We both like him so much. He proudly showed us his university, and it is one he may well be proud of. The next day he took us to the Ghezirah Country Club for tea—at least, he took me. Dad had what Dr. Watson called “the Gippy tummy,” so that is another foreign ailment added to our “Spanish flu” and “Turkish grip.”

The first two occurrences of the variant gyppy stomach (also gippy stomach, gyppie stomach) that I have found are as follows:

1-: From the column Uncle Ray’s Corner, by Ramon Coffman (1896-1989), published in several U.S. newspapers in January 1938—for example in The Austin American (Austin, Texas, USA) of Sunday the 2^nd:

Cairo is a big city and worth while to visit but it can not be called a clean city. People say it is “better than it used to be,” but even if it is, it is far from having the standards we could wish.
There is too much dust. The streets (with their heavy traffic of donkeys and other animals) are not cleaned often enough. Most parts of the city are crowded with too many people. Above all, there are large numbers of flies and mosquitoes at certain seasons of the year.
Flies and mosquitoes spread disease in one way or another. There is “gyppie stomach,” for instance, also “dengue fever.” Neither of these, it seems, is ever the cause of death, but both make life appear not so well worth living until they are gone. I have been careful about my food and drink, and have guarded against mosquitoes as well as possible. So far, I have escaped falling victim to any illness in Egypt, but some visitors have not had such good fortune.

2-: From Dorothy Scheller To Aid U.S. Ambassador To El Salvador, by Barbara Fark, published in The Indianapolis Star (Indianapolis, Indiana, USA) of Sunday 25^th January 1953:

Miss Dorothy Scheller, who joined the Foreign Service for a change, next month will become secretary to the United States ambassador to El Salvador.
[…]
[…] Post reports from San Salvador indicate “tight” housing conditions and high prices. Both situations contribute to a cost of living at least as high as that in Indianapolis.
The reports also warn against eating fruits and vegetables which will bring on a malady, sometimes known as “gippy stomach,” common at one time or another to all Americans employed abroad. This “spells the Near East” to Dorothy who spent her first government years in Jerusalem and Ankara.