Its a Slur that are commonly used against East/Southeast asians mostly Japanese People( 日本人) because the word “yellow” has been referred to Asians for a long time while “Monkey” is used against Japanese people because people say that they look like monkeys, and That is why “Yellow Monkey” is a slur.
this is most likely the worst insult known. it refers to someone of oriental background; however it can be used to refer to anyone who has no idea what the hell is going on.