A woman who is alone because of divorce, separation, rejection, etc
[1839+; because her husband is still above the grass rather than under it]
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with grass widow
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.