WordNet (r) 3.0 (2006):

back of beyond
    n 1: a very remote and inaccessible place; "you'd have to go to
         the back of beyond to find one of those"

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "back of beyond":
   China, Darkest Africa, God knows where, Greenland, North Pole,
   Outer Mongolia, Pago Pago, Pillars of Hercules, Siberia,
   South Pole, Thule, Tierra del Fuego, Timbuktu, Ultima Thule, Yukon,
   antipodean, antipodes, back, back-country, backwood, backwoods,
   backwoodsy, frontier, godforsaken, godforsaken place, hinterland,
   hyperborean, inaccessible, jumping-off place, nowhere,
   out of reach, out-of-the-way, outback, outer space, outpost,
   outskirts, pole, sylvan, the Great Divide, the South Seas,
   the boondocks, the moon, the sticks, the tullies, unapproachable,
   ungetatable, untouchable, up-country, virgin, waste, wild,
   wilderness, woodland