Green’s Dictionary of Slang

(US) diarrhoea; thus the GI pill, a pill designed to combat the malady.

1944		P. Kendall Dict. Service Sl. n.p.: I’ve got the G I’s . . . gastronomic insurrections.
1946		J.W. Bishop ‘American Army Speech’ in AS XXI:4 Dec. 247: The G.I.’s is – or are – diarrhea; the phrase is a contraction, probably, of ‘the G.I. trots.’ The malady is cured or aggravated by a G.I. pill.
1946		N.Y. Herald Trib. 3 Jan. 12/6: The ‘G. I. bug’ (gastro intestinal), as the girls at Smith College call the prevalent ‘flu’ germ, hit the nice couple who run the Farm Home Food Delicatessen in Watertown, Wis.
1952		N.Y.Herald Trib. 22 Feb. 15/6: Most tourists complain that, sooner or later, they get the ‘tourist disease,’ which in the Army was known as ‘G. I. trots’.
1953		L. Uris Battle Cry (1964) 332: I got some British Navy rum and had the G.I. craps for a week.
1956		‘Ed Lacy’ Men from the Boys (1967) 28: I think I have a case of the old G.l.’s.
1961		T.I. Rubin In the Life 82: Doc! What a case of the GI’s! I been in and out of the crapper ten times already.
1966		Hope Star (AR) 23 June 3/5: Turista is the diarrhea of travelllers [...] also called the Aztec two-step [...] and the GI trots.
1977		L. Heinemann Close Quarters (1987) 17: Cures heartburn, jungle rot, the Gee-fucken-Eyes, all them things.
1984–5		Maledicta VIII 99: Leitner and Lanen list G.I.s as synonymous with the trots. Wentworth and Flexner see this as a shortening of G.I. shits and suggest that it is an abbreviation of ‘gastrointestinal’.
1990		E. Ruggero 38 North Yankee 243: Twenty-five soldiers in the company had the ‘GI trots’.
2004		A. Giambroni Odor Of War 28: The second night at this depot, the majority of the new replacements developed the ‘GI Trots,’ known as diarrhea in civilian life.