grass widow (n.) Look up grass widow at Dictionary.com
1520s, originally "discarded mistress" (compare 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]