While the phrase "chirk up" usually means to brighten up or cheer up, there are several antonyms to this phrase. One of them is "mope," which means to move or act in a listless or gloomy manner. Another antonym for "chirk up" is "sadden," which means to cause or experience sorrow or unhappiness. A third antonym is "depress," which means to make someone feel discouraged or low in spirits. Lastly, "dishearten" is also an antonym for "chirk up," which means to cause someone to lose confidence or enthusiasm. So, while there are several ways to describe someone feeling happy or cheerful, there are also several ways to describe someone feeling sad or discouraged.