The word boondocks is a relic of American colonialism. British English imported lots of words from its far-flung colonial possessions, but American colonial aspirations mainly produced words derived from Spanish and adopted with the settling of the West. This one, however, is an exception.
It derives from the Tagalog word bundok, meaning mountain. It was adopted into the language by occupying American soldiers in the Philippines as a word meaning any remote and wild place. By 1909, only some ten years after the American conquest of the islands, the word had caught on enough to rate an entry in that year’s Webster’s New International Dictionary. Despite this, however, it remained primarily a military slang term, especially among Marines, until the 1960s, when, probably because of the Vietnam War, it gained wider, civilian usage.