"they will flair out from the waist instead of the hip"

I think you mean the bottom of the abdomen instead of the waist. I don't know if there's a specific term for "bottom of the abdomen" in the same way that the waist is the part of the abdomen between the ribcage and the pelvis. The separation between the shirt and the trousers in men's clothing happens at the bottom of the abdomen, in contrast to women's clothing, which happens at the waist.

Also dresses today start flairing out from the waist, none start flairing out from the hip.