The Collaborative International Dictionary of English v.0.48:

Unknown \Un*known"\, a.
   Not known; not apprehended. -- Un*known"ness, n. [R.]
   --Camden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unknown
    adj 1: not known; "an unknown amount"; "an unknown island"; "an
           unknown writer"; "an unknown source" [ant: known]
    2: being or having an unknown or unnamed source; "a poem by an
       unknown author"; "corporations responsible to nameless
       owners"; "an unnamed donor" [syn: nameless, unidentified,
       unknown, unnamed]
    3: not known to exist; "things obscurely felt surged up from
       unknown depths"
    4: not famous or acclaimed; "an obscure family"; "unsung heroes
       of the war" [syn: obscure, unknown, unsung]
    5: not known before; "used many strange words"; "saw many
       strange faces in the crowd"; "don't let anyone unknown into
       the house" [syn: strange, unknown]
    n 1: an unknown and unexplored region; "they came like angels
         out the unknown" [syn: unknown, unknown region, terra
         incognita]
    2: anyone who does not belong in the environment in which they
       are found [syn: stranger, alien, unknown] [ant:
       acquaintance, friend]
    3: a variable whose values are solutions of an equation [syn:
       unknown, unknown quantity]

Moby Thesaurus II by Grady Ward, 1.0:

146 Moby Thesaurus words for "unknown":
   Heaven, Paradise, a better place, able, abstruse, afterlife,
   afterworld, anonymous, beclouded, blind, buried, close, clouded,
   concealed, covered, covert, cryptonymic, cryptonymous, dark,
   dark horse, destiny, eclipsed, enigma, enigmatic, eternal home,
   fate, frontier, frontiers of knowledge, future state, hid, hidden,
   home, humble, ignotus, in a cloud, in a fog, in eclipse, in purdah,
   in the wings, incalculable, incognito, incognizable, incommunicado,
   inglorious, innominate, latent, life after death, life to come,
   little known, matter of ignorance, mysterious, mystery, n,
   nameless, next world, no credit to, obfuscated, obscure, obscured,
   occult, otherworld, postexistence, puzzle, puzzling, recondite,
   renownless, riddle, sealed, sealed book, secluded, secluse, secret,
   sequestered, strange, terra incognita, the beyond,
   the good hereafter, the grave, the great beyond,
   the great hereafter, the hereafter, the incalculable, the strange,
   the unfamiliar, the unknowable, the unknown, unacknowledged,
   unapparent, unapprehended, unascertained, unbeknown, uncelebrated,
   uncharted, unclassified, undefined, under an eclipse, under cover,
   under house arrest, under wraps, underground, undesignated,
   undisclosed, undiscoverable, undiscovered, undistinguished,
   undivulged, unexplained, unexplored, unexplored ground,
   unexplored territory, unexposed, unfamed, unfamiliar, unfathomed,
   unglorified, unheard, unheard-of, unhonored, unidentified,
   uninvestigated, unknowable, unknown quantity, unnamed, unnotable,
   unnoted, unnoticed, unperceived, unplumbed, unpopular,
   unrecognized, unremarked, unrenowned, unrevealed, unspecified,
   unsung, unsuspected, untold, untouched, virgin, what bodes,
   what is fated, without a name, world to come, wrapped in clouds, x,
   z



Bouvier's Law Dictionary, Revised 6th Ed (1856):

UNKNOWN. When goods have been stolen from some person unknown, they may be
so described in the indictment; but if the owner be really known, an
indictment alleging the property to belong to some person unknown is
improper. 2 East's P. C. 651 1 Hale, P. C. 512; Holt's N. P. C. 596 S. C. 3
Eng. Common Law Rep. 191; 8 C. & P. 773. Vide Indictment; Quidam.