Having a flat chest; having small breasts.(adjective)
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Use "flat-chested" in a sentence
"The flat-chested“flapper” style of female physique declined in popularity, and the foundation undergarment industry experienced a period of dramatic growth."
"The hourglass cut of the dress is flattering and sexy and that plunging neckline can either be flat-chested flapper-girl demure (Kershaw) or Sophia-Loren-style curvaceous (Crawford)."
"“It means we need to tie down your breasts so you appear flat-chested,” the wardrobe woman explained."