Rashi on Deuteronomy 21:12/רש"י על דברים כ״א:י״ב

1ועשת את צפרניה means, AND SHE SHALL LET GROW HER NAILS — She must let them grow, so that she should becomes repulsive to her captor (cf. Sifrei Devarim 212:4; Yevamot 48a; see also Rashi on Genesis 1:7 and our Note thereon).