The word 经济 is from 1500 years ago, which means 经国济民 (to govern and manage the country and to relieve common people). During the Meiji Period, Japanese scholars started to use this word to indicate all the abilities necessary to maintain social life, including producing, consuming, and trading. The two meanings were so different between China and Japan at that time. After, a famous translator called 梁启超 started to introduce a way that translated "political economy" into 经济, which replaced the old translations such as 生计学、平准学 and has been continuously cited until now. Of course, there still exist dissenters towards the way.

Despite the fact that it has existed in the history of China for thousand years, but actually it's been used to infer to economy since the last century, borrowed from Japanese. In the past, 经济 means "to manage a country" {经 means 经营to run/manage, 济 means 救济 to help those who are in trouble, in poor condition}. While in the late Qing dynasty, China was overwhelmed by the Occident, so was Japan. And Japan turned his eyes to the Western, when 经济 was given a new meaning, a word used to specificly translate "economy". Advanced people in China began to translate books of the developed countries when at this time Japan had been preceding China in many aspects. In such circumstance, lots of books, contents from the western, but of Japanese language, were introduced into China. This word 经济 got to become popular, until today. I hope I have got myself clear enough :)