Green’s Dictionary of Slang

(US campus) to neck, to pet or make love in an automobile; thus boondocking n.

1960, 1975		Wentworth & Flexner DAS.
1969		Current Sl. III:4 4: Boondock, v. To take beer, brod and blanket to the woods.
1989		H. Rawson Dict. of Invective (1991) 56: Among the younger set, boondocking also has become something of a euphemism for the petting that often takes place when couples retire to isolated places.