Car key parties are one of those quintessential 70's phenomena like pass-through windows, fondue kits and sodastreams that only people too young to have experienced them (me included) feel nostalgic for. If I could go back in time, the 1970's would be high on my list.
"Midge Lackie, whose early days as an army wife in Aden had prepared her for almost any surprise, was nonetheless shocked when an acquaintance, a corporal's wife, was evicted from her quarter in Minden in the 1970s. She was sent back to her parents' home in Austria because it transpired she had been holding car key parties while her husband was away."
—Annabel Venning, Following the Drum: The Lives of Army Wives and Daughters Past and Present (London: Headline, 2005), 159
I had no earthly clue what this was, even after asking several other people. A quick Google search reveals it's a "party" at which all the men (at this time, anyway) would throw their keys into a bowl, the women would pull a set out, and go home/have sex/roast marshmallows or something with the guy whose keys she pulled.
... Gross. (Everyone here knows how much I hate marshmallows.)