In ancient times, houses were laid out (安排) in a certain way. The door of the home would face south, with the bathroom located (位于) in the northeastern part of the house and the kitchen located in the southeastern part.

中国古代的房子大都坐北朝南，厕所多位于东北角，而厨房则多位于东南。

On maps, north is usually at the top of the map, while south is at the bottom (底部). In Chinese, we say “上北下南” to refer to the north as shang (upper) and the south as xia (lower). So people would say they were going “up” to the bathroom and “down” to the kitchen because of how their homes were laid out.

上北下南，左西右东……这下知道为啥说“上厕所”、“下厨房”了吧！

What’s more, people thought cooking was humble (卑微的) work. Going to the kitchen would lower their position (地位). 

还有一点，古代人们觉得“做饭”是一个“卑微”的事情，去厨房会降低身价，所以会用“下”这个字。但现在大家说到这个词已经不会有这个联想啦！