It is so hard to remember this kanji by its origin because it changed too much. However, I will think of a way to help you remember it.
昼 = 尺 + 旦。
尺 means ruler. 旦 looks like a sun above the line. To express that the sun is still there. How do you know that it is afternoon now while you don’t have a watch? See the sun.
So 昼 stands for “afternoon”.