WordNet (r) 3.0 (2006):

Timbuktu
    n 1: a city in central Mali near the Niger river; formerly
         famous for its gold trade

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "Timbuktu":
   China, Darkest Africa, God knows where, Greenland, North Pole,
   Outer Mongolia, Pago Pago, Pillars of Hercules, Siberia,
   South Pole, Thule, Tierra del Fuego, Ultima Thule, Yukon,
   antipodes, frontier, godforsaken place, jumping-off place, nowhere,
   outback, outer space, outpost, outskirts, pole, the Great Divide,
   the South Seas, the boondocks, the moon, the sticks, the tullies