The Collaborative International Dictionary of English v.0.48:

Hyperborean \Hy`per*bo"re*an\, n.
   1. (Greek Myth.) One of the people who lived beyond the North
      wind, in a land of perpetual sunshine.
      [1913 Webster]

   2. An inhabitant of the most northern regions.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Hyperborean \Hy`per*bo"re*an\, a. [L. hyperboreus, Gr. ?; "ype`r
   over, beyond + ?. See Boreas.]
   1. (Greek Myth.) Of or pertaining to the region beyond the
      North wind, or to its inhabitants.
      [1913 Webster]

   2. Northern; belonging to, or inhabiting, a region in very
      far north; most northern; hence, very cold; fright, as, a
      hyperborean coast or atmosphere.
      [1913 Webster]

            The hyperborean or frozen sea.        --C. Butler
                                                  (1633).
      [1913 Webster]

WordNet (r) 3.0 (2006):

Hyperborean
    n 1: (Greek mythology) one of a people that the ancient Greeks
         believed lived in a warm and sunny land north of the source
         of the north wind

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "hyperborean":
   Siberian, algid, antarctic, antipodean, arctic, austral,
   back of beyond, below zero, biting, bitter, bitterly cold, bleak,
   boreal, brisk, brumal, cold, cold as charity, cold as death,
   cold as ice, cold as marble, crisp, cutting, east, eastbound,
   easterly, eastermost, eastern, easternmost, freezing,
   freezing cold, frigid, gelid, glacial, godforsaken, hibernal,
   hiemal, ice-cold, ice-encrusted, icelike, icy, inaccessible,
   inclement, keen, meridional, nipping, nippy, north, northbound,
   northeast, northeasterly, northeastern, northerly, northern,
   northernmost, northwest, northwesterly, northwestern, numbing,
   occidental, oriental, out of reach, out-of-the-way, penetrating,
   piercing, pinching, raw, rigorous, severe, sharp, sleety, slushy,
   snappy, south, southbound, southeast, southeasterly, southeastern,
   southerly, southern, southernmost, southwest, southwesterly,
   southwestern, stone-cold, subzero, supercooled, unapproachable,
   ungetatable, untouchable, west, westbound, westerly, western,
   westernmost, winterbound, winterlike, wintery, wintry