From China Digital Space
天朝 (Tiān cháo): Celestial Empire
The Celestial Empire is an ancient name for China. Netizens use the term sarcastically to refer to China under the current government. It often suggests that China’s leaders are self-important and have a China-centric view of the world.
“Celestial Empire” is also used in other sarcastic plays on words, such as Celestial Empire goes into the pot (天朝上锅 Tiān cháo shàng guō), which is a near-homonym for “Celestial Empire above all other nations.”
Probably because “Celestial Empire” is often used in criticism of the government or state of affairs in China, an alternate way of writing it has at times been blocked from Sina Weibo search results.
TG is sometimes used as an approximate abbreviation, but it actually stands for 土共 Tǔgòng, another name for the Communist Party of China.