colloquial shortening of boondocks "remote and wild place;" by 1964, originally among U.S. troops in Vietnam War (in reference to the rural areas of the country, as opposed to Saigon).
Entries linking to boonies
boondocks n.
"remote and wild place," 1910s, from Tagalog bundok "mountain." A word adopted by occupying American soldiers in the Philippines for "remote and wild place." It was reinforced or re-adopted during World War II. Hence, also boondockers "shoes suited for rough terrain," originally (1944) U.S. services slang word for field boots.