cohabitation (n.)

mid-15c., cohabitacioun, "action or state of living together," from Old French cohabitacion "cohabitation; sexual intercourse," or directly from Late Latin cohabitationem (nominative cohabitatio), noun of action from past participle stem of cohabitare "to dwell together," from co- "with, together" (see co-) + habitare "to live, inhabit, dwell," frequentative of habere "to have, to hold, possess" (from PIE root *ghabh- "to give or receive"). Specifically "state of living together as husband and wife without benefit of marriage," implying sexual intercourse, from 1540s.