The phrase "to nip the waist" means to tighten or make the waist area smaller, usually through the use of clothing or accessories like belts or corsets. Full definition
The 1940s also made their presence felt in 3/4 - sleeved coats and a black suit — fitted jacket with nipped waist and flaring skirt — which Erdem labelled «Dior - ish». (businessoffashion.com)
You can knot it as loosely or tightly as you like while still maintaining the appearance of nipping your waist. (redbookmag.com)
These include (but are not limited to): (1) the perfect little black dress (paired with the the perfect red lipstick), (2) killer heels that put a swing in your hips and a raise of your eyebrow, (3) a vintage jacket to nip the waist in something fierce, (4) a sundress to twirl in and (5) the comfiest pajamas (I firmly believe in this last one. (thedressedaesthetic.com)