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1520s, originally "discarded mistress" (cf. German Strohwitwe, literally "straw-widow"), probably in reference to casual bedding. Sense of "married woman whose husband is absent" is from 1846.
[G]rasse wydowes ... be yet as seuerall as a barbours chayre and neuer take but one at onys. [More, 1528]
A woman who is alone because of divorce, separation, rejection, etc
[1839+; because her husband is still above the grass rather than under it]