洪荒之力 (Hónghuāng zhī lì)
In 2015, in a popular Chinese drama called 《花千骨》, 洪荒之力 was used to describe the strongest power in the drama, power that is so strong that it can destroy the world. However, in 2016, during the Rio Olympic Women's 100m backstroke semi-final, the Chinese swimmer Fu Yuanhui said in an interview: "我已经用了洪荒之力” with a comedic expression and the internet immediately caught on.
Therefore, it is now used to describe that you have put in the upmost effort and have exhausted your powers.
Breaking down the phrase:
洪: refers to water
荒: refers to the plain fields
之力: refers to power within your means