Brewer's: Grass Widow
was anciently an unmarried woman who has had a child, but now the word is used for a wife temporarily parted from her husband. The word means a grace widow, a widow by courtesy. (In French, veuve de grace; in Latin, viduca de gratia; a woman divorced or separated from her husband by a dispensation of the Pope, and not by death; hence, a woman temporally separated from her husband.)
“Grace-widow (`grass-widow') is a term for one who becomes a widow by grace or favour, not of necessity, as by death. The term originated in the earlier ages of European civilisation, when divorces were granted [only] by authority of the Catholic Church.” —Indianopolis News (1876).
The subjoined explanation of the term may be added in a book of “Phrase and Fable.” During the gold mania in California a man would not unfrequently put his wife and children to board with some family while he went to the “diggins.” This he called “putting his wife to grass,” as we put a horse to grass when not wanted or unfit for work.