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The G.I.s means basically to have diarrhea. This saying was probably given the name as I am told soldiers stationed in china in WWII got diarrhea when eating local food.
"Johnny can't come out to play. He's got the G.I.s."
"Eating food in China gave Sgt. Hollis the G.I.s, although he said it was better than eating K-Rations."
by Macsotsu January 30, 2005