CACTUS
In my snippet about the Australian expression last time, I noted that there were no native cacti in that country. John Weiss pointed out that the prickly pear had been imported from the US in the early 1800s as stock fodder but had become a serious invasive pest in New South Wales and Queensland by the latter part of the nineteenth century. It was so well known he feels the expression was most certainly native. Many Australians wrote to make the same point Rob Coates did:

• "Sometimes the single word 'cactus' is used but it's generally recognised to be a shortening of 'cactus fuctus'.

• This is said as a pseudo-Latin phrase to bring a touch of wry humour to an otherwise unfortunate situation. For example, a mechanic, after inspecting the starter motor in your car might announce 'No wonder it won't start, mate - this is cactus fuctus!'" (An alternative spelling with the "k" in place is also common, I gather.)

Source: World Wide Words Michael Quinion, 2 Aug 09