A pejorative termed used by overseas Chinese in Southeast Asia to describe mainland Chinese immigrants in the same way FOBs is used by Asians in the US, Canada and Australia to disparage recent Asian immigrants.
The etymology of the word stems from the Hokkien (a predominant Chinese dialect spoken in Southeast Asia) pronunciation of China "Tiong Kok" which in standard Mandarin would be pronounced as Zhong Guo.
The use of the word is usually preceded by other adjectives ranging from rude, chao and arrogant to hot, sexy and chio - with the latter three being generally reserved for working ladies of PRC origins. As demonstrated from the above, the use of the term Ah Tiong transcends gender boundaries.
That chao ah tiong is damn fucking rude! How dare he cut our queue! We're in KL, not Kunming!