“It’s cold in spring, but then it’s cold again when it’s warm at first” is a saying that describes the changeable temperatures in spring. It means that although the weather has turned warm in spring, there are still bursts of chill. The weather is unpredictable and people need to pay special attention to keeping warm.
The origin of "The cold in spring is sharp, and the sudden warmth returns to cold" is more complicated and may be a combination of multiple documents. Among them, "The spring cold is steep and solemn, and the remaining snow has not disappeared" in "Book of Jin: Biography of Wang Yuan" and "Warmth returns to cold" in "Book of Sui: Biography of Xu Shiji" have certain influence. In the "Book of the Later Han: Biography of Ma Yuan", there is also a record of "the sun is warm in winter, but sometimes there is cold air". Through the inheritance of time, these documents and records gradually evolved into the idioms that are widely circulated today.