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]

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.