The headword -- evidently quoted from somewhere -- is the feminine nominative singular of the adjective mnhsto/s, mnhsth/, mnhsto/n, from mna/omai (mu 1149, mu 1177); see LSJ entry at web address 1.
The first part of the primary gloss is also found in the entry for mnhsth/ in Hesychius mu1511. The second part of the Suda's entry differs from the entry in Hesychius, which continues: "or she who has remained fasting from virginity, that is to say, she has not known man ( h)\ h( e)k parqenei/as mei/nasa nh=stis, h)/goun a)pei/randros).
See also mu 1168.