Words still fail when it comes to describing unmarried people living together. In the 19th century they did it without benefit of clergy, a phrase that originally described the denial of the last rites to one who had committed suicide. That makes sense. You might as well as have been dead, living in sin as you were.
Even into the 20th century, such cohabitation would have been considered the living end (late 1930s), "quite extraordinary, outrageous, and scandalous." We use the phrase today to refer to the utmost in any situation: "Her 'bod' was the living end." "His gross references to her were the living end."
By the roaring twenties, things had loosened up somewhat. Traveling salesmen and truckers began shacking up with local girls to meet their needs while away from home. Shack? Well, you can just imagine the facilities. Soldiers during WWII did likewise — to such a point that people began associating shacking up with promiscuity.
It's now relatively easy to shack up and still earn the good housekeeping seal of approval. You have but to enter into a primary relationship with a significant other (c.1980s). For one so significant, however, the other for the most part remains somewhat non-descript.
Politics in the form of the U.S. Census Bureau took a stab at it — making for the strangest bedfellow of all. After years of struggling with "partner" and "roommate," it finally came up with the POSSLQ, a "person of the opposite sex sharing living quarters." Overlooked in the process was the PSSSLQ.
Either way, have you hugged your POSSLQ or PSSSLQ today? That indeed is the living end!