it's a euphemism from Detroit MI. There was an island in the Detroit River that had a public park on it that was infrequently patrolled by police because it was too far for them to be arsed to bother, called Grosse Isle. There were several scenic overlooks one could look out over the water or have a picnic at and since they were generally secluded, there were no gates to close it off at night, teenagers would take their dates there to make out back between the 1940's and 1960's. (souce, my dad, who grew up in the downriver area) When they were caught, this became a popular "excuse/explanation" to give to the poe-poe/parents.