Usually a euphemism is invented where speaking directly of a subject is considered taboo. In Australia euphemisms are often invented for the sheer colour and humour of the thing. Three activities here in particular have a plethora of euphemisms. Those are urination, vomiting and sex.
Shag is used popularly in Australia as well as in the United (ahem) Kingdom, though root is more local. To shag is to have sex.