Green’s Dictionary of Slang

(Aus.) Mediterranean gut-ache, an ‘illness’ supposedly contracted by Greek, Yugoslav and similar ‘Mediterranean’ immigrants to Aus., seen as innately lazier than their Anglo-Saxon counterparts.

1982

N. Keesing Lily on the Dustbin 51: There are racist overtones to [...] the ‘Mediterranean back’, which denotes derisive suspicion about a back-ache and ‘Mediterranean gut-ache’, usually shortened to ‘MGA’ which is just as suspect as the back-ache.