one's middle = talia, pas

This belt actually IS a corset

Source: http://www.stylehive.com/bookmark/etsy-kvodesign-exquisite-black-leather-corset-belt-waist-cincher-custom-made-to-your-size-518356

talia, pas

English sentence and Polish translation

If you put a belt in your middle, the dress will look better. = Jeśli założysz pasek w talii, sukienka będzie wyglądać lepiej.