I’m not sure why ‘1200’ became the magic number of calories women should consume if they want to lose weight

I’m not even sure how I know of this number. I just seem to know it, and my friends know it, and my mum knows it too. Somehow, somewhere, I was taught that if I want to have a flat stomach and tight bottom, I need to limit my calorie intake to 1200 a day. And take up cardio.