"chaste, pure, virgin," 1590s, originally (early 15c.) "belonging to or dedicated to Vesta," Roman goddess of hearth and home, from Latin vestalis. The noun is recorded from 1570s, short for Vestal virgin, one of four (later six) priestesses (Latin virgines Vestales) in charge of the sacred fire in the temple of Vesta in Rome. From 1580s in reference to any virgin or chaste woman.
They entered the service of the goddess at from six to ten years of age, their term of service lasting thirty years. They were then permitted to retire and to marry, but few did so, for, as vestals, they were treated with great honor, and had important public privileges. Their persons were inviolable, any offense against them being punished with death, and they were treated in all their relations with the highest distinction and reverence. A vestal who broke her vow of chastity was immured alive in an underground vault amid public mourning. There were very few such instances; in one of them, under Domitian, the chief of the vestals was put to death under a false charge trumped up by the emperor. [Century Dictionary]