1
general::
A woman who is separated from her husband, either by divorce or temporary absence. For example, She's a grass widow these days, with Herb traveling to golf tournaments all over the country. The expression dates from the 16th century, when it referred to the mother of an illegitimate child, grass presumably alluding to the open-air setting of the child's conception.
American Heritage Idioms
2
general::
Phrase(s): grass widow
a woman abandoned by her husband. (The origin of this is not clear.) • Jane’s husband isn’t dead, but she’s a widow just the same—a grass widow. • Bill ran off and left Mary a grass widow.
McGrawhill's American Idioms And Phrasal Verbs