The term handmaid of God is an expression found in patristic literature.
[1] In early Christian inscriptions the title ancilla Dei (Latin for "handmaid of God") is often given to a deceased woman.
This informs us that a certain monument was erected by a husband to his wife, whom he styles Dei ancilla: "(Laur)entius Rufine coniugi Dei anci(llae)...".
This reference shows that even in the sixth century, ancilla Dei is a title not peculiar to religious persons; the author regarded it as necessary to state explicitly that she was nonnes.
[3] From the pontificate of Pope Gregory I (590–604), however, only nuns, as a rule, were qualified by this title.