According to Mirriam-Webster, a grass widow can mean:
- a discarded mistress
- a woman who has had an illegitimate child
- a woman whose husband is temporarily away from her
- a woman divorced or separated from her husband
It feels very archaic... and it is. The first known use was in 1699 (the illegitimate child).
I’ve also seen more modern definitions in which the grass is a metaphor for a golf green, where one’s spouse might disappear to.