The Collaborative International Dictionary of English v.0.48:

Unapproachable \Unapproachable\
   See approachable.

WordNet (r) 3.0 (2006):

unapproachable
    adj 1: discouraging intimacies; reserved; "an unapproachable
           executive" [ant: approachable]
    2: inaccessibly located or situated; "an unapproachable chalet
       high in the mountains"; "an unreachable canyon"; "the
       unreachable stars" [syn: unapproachable, unreachable,
       unreached, out of reach(p)]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "unapproachable":
   Olympian, aloof, antipodean, austere, back of beyond, backward,
   bashful, beyond compare, beyond comparison, beyond reach, blank,
   chilled, chilly, closed forever to, closed to, cold, constrained,
   cool, detached, discreet, distant, easily first, exclusive,
   expressionless, facile princeps, forbidding, frigid, frosty,
   godforsaken, guarded, hyperborean, icy, immortal, impassive,
   impenetrable, impersonal, impervious, inaccessible, incomparable,
   inimitable, insociable, introverted, invincible, lost to,
   matchless, modest, nulli secundus, offish, out of reach,
   out-of-the-way, peerless, remote, removed, repressed, reserved,
   restrained, reticent, retiring, seclusive, second to none,
   shrinking, standoff, standoffish, subdued, sui generis, suppressed,
   un-come-at-able, unaccessible, unaffable, unapproached,
   unattainable, unavailable, unbeatable, unbending, uncommunicative,
   uncongenial, undemonstrative, undiscoverable, unequaled,
   unexampled, unexcelled, unexpansive, unfriendly, ungenial,
   ungetatable, ungettable, unique, unmatchable, unmatched,
   unobtainable, unparagoned, unparalleled, unpeered, unprocurable,
   unreachable, unrivaled, unsecurable, unsurpassable, unsurpassed,
   untouchable, withdrawn, without equal, without parallel