femme de petite vertu

Etymology
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Noun

 * 1)  loose woman, strumpet promiscuous woman
 * 2)  woman of easy virtue, prostitute woman having sex for profit
 * 1)  woman of easy virtue, prostitute woman having sex for profit

Usage notes
In addition to the usual plural, which preserves the set form of the locution , the alternative is attested, but significantly less common.