Perhaps in allusion to a bed of grass or hay
The term grass widow cries out for explanation of what grass means and how grass widow came to have its varied though related senses. Grass probably refers to a bed of grass or hay as opposed to a real bed. This association would help explain the earliest recorded sense of the word (1528), "an unmarried woman who has lived with one or more men," as well as the related senses "an abandoned mistress" and "the mother of an illegitimate child." Later on, after the sense of grass had been obscured, people may have interpreted grass as equivalent to the figurative use of pasture, as in out to pasture. Hence grass widow could have developed the senses "a divorced or separated wife" or "a wife whose husband is temporarily absent."