Boondocks is a plural noun, with no singular form, that means an extremely rural area, usually without many of the conveniences of a metropolitan city. It can also be a landscape with harsh terrain and lots of dense foliage. The term is most often found in the phrase in the boondocks or out in the boondocks.
Boondocks comes from the Tagalog language, specifically bundok, which means mountain. The term is found most often in the United States, but is gaining ground in other countries as well.
Boonies is simply a shortened version of boondocks and is used once for every three instances of the original term.
Both versions of the term are exclusively used with the article the, as in the boondocks and the boonies; unless the word is used as a title, name, or other proper noun.
“One was Domaine Chandon, in Napa, and one was in Michigan in the boondocks,” he said. [The Wall Street Journal]
“I honestly thought it would only be shown out in the boondocks and no-one would ever see it.” [The Independent]
The documentary “My Way” follows obscure rocker Rebekah Snyder-Starr and her backup singer-tambourine player Annika Alliksoo as they hit the road from the boondocks of Kittanning, Pa., to the Sunset Strip in 2010. [Los Angeles Times]
“On average, that runs between $600 and $700, depending on the location in the United States and if we have to send an airplane out into the boonies.” [The New York Times]