From China Digital Space
天朝 (Tiāncháo): Celestial Empire
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, have a China-centric view of the world, and embody many of the imperial trappings communism was supposed to have thrown off.
“Celestial Empire” is also used in other sarcastic plays on words, such as “Celestial Empire goes into the pot” (天朝上锅 Tiāncháo shàng guō), which is a near-homonym for “Celestial Empire above all other nations.”
Zhenshidehuangyane (@真实的谎言e): Special characteristics of the Celestial Empire: corruption is its father, shamelessness is its mother! What a mystical country! (September 24, 2014)
天朝特色，腐败是爹，无耻是妈！ //@老王故事会://@相思洋: 神奇的国度！[Chinese]
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.