Cantonese cuisine involves xx willow vegetables, which generally refers to cutting meat into strips for cooking.
As for the origin of the word "beef willow", I'm afraid I haven't done any historical research. It may be taken from the long and narrow shape of "willow leaf". Usually we talk about beef tenderloin, which is filet mignon (definitely not sirloin! That's sirloin! ! There is only one cow, which is slender in shape and slightly small and pointed at one end, just like a willow leaf. Therefore, it is very appropriate to call it "beef fillet".