The Collaborative International Dictionary of English v.0.48:

Prehistoric \Pre`his*tor"ic\, a.
   Of or pertaining to a period before written history begins;
   as, the prehistoric ages; prehistoric man.
   [1913 Webster]

WordNet (r) 3.0 (2006):

prehistoric
    adj 1: belonging to or existing in times before recorded
           history; "prehistoric settlements"; "prehistoric peoples"
           [syn: prehistoric, prehistorical]
    2: of or relating to times before written history; "prehistoric
       archeology"
    3: no longer fashionable; "my mother has these prehistoric ideas
       about proper clothes"

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "prehistoric":
   aboriginal, ancestral, ancient, antediluvian, antepatriarchal,
   antiquated, atavistic, autochthonous, earliest, early, erstwhile,
   fore, former, fossil, humanoid, immemorial, late, old,
   old-fashioned, olden, once, onetime, passe, past, patriarchal,
   preadamite, preglacial, prehuman, previous, primal, prime,
   primeval, primitive, primogenial, primoprimitive, primordial,
   prior, pristine, protohistoric, protohuman, quondam, recent,
   sometime, then



The Devil's Dictionary (1881-1906):

PREHISTORIC, adj.  Belonging to an early period and a museum.
Antedating the art and practice of perpetuating falsehood.

    He lived in a period prehistoric,
    When all was absurd and phantasmagoric.
    Born later, when Clio, celestial recorded,
    Set down great events in succession and order,
    He surely had seen nothing droll or fortuitous
    In anything here but the lies that she threw at us.
                                                         Orpheus Bowen