When the Chinese character for "summer" was originally created, the pictograph depicts a human figure, like the image shown below, which can be found on the so-called oracle bones that date back over 3,000 years from our time.
Slightly later, the character evolved into something like this:
which can be found in the inscriptions on bronze vessels. In the Qin period over 2,000 years ago, the character took the shape like this:
Since then the character has been standardized as we see it today:
So the character in question started as the depiction of a human figure. It was actually a word very ancient Chinese used to refer to themselves. That is, they called themselves the 夏 [xià] people. There is an ongoing debate on whether a historical state, a dynasty, known as 夏 [xià] actually existed. The answer the question, in a way, depends on how you define "state" or "dynasty." Regardless, in very early times the Chinese referred to themselves as the Xia people. Today the Chinese still proudly use the term huá xià zǐ sūn - "descendants of the Great Xia."
But how did the word that originally meant a man or a people come to also mean "summer"? There are a couple theories. According to one explanation, the original pictograph depicts a person who kneels under the sun, and the image is used to refer to the idea of "hot" or "summer." According to another theory, the original pictograph depicting "man" or "people" had same pronunciation as the spoken word for "summer," the character 夏 was used to refer to summer. As result, the character 夏 has two main meanings, "ancient Chinese" and "summer."
One ancient saying that contains the word 夏 is “Xià chóng bù kěyǐ yǔ bīng” - "summer bugs cannot be told about ice." In other words, it is useless to try to talk about ice with summer bugs. The reason is evidently that summer bugs have such short life spans, they have no concepts of winter, ice and the like. This is actually rather profound. Think how little we humble (or no so humble) human beings know about the universe.