WordNet (r) 3.0 (2006):

know nothing
    n 1: an ignorant person [syn: ignoramus, know nothing,
         uneducated person]

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "know nothing":
   antiblack, awkward, be green, be ignorant, benighted, biased,
   blankminded, callow, chauvinistic, colored, dabbler, dilettante,
   dimwit, doctrinaire, dogmatic, dolt, dope, dumb, dummy, dunce,
   empty, empty-headed, fool, gauche, green, greenhorn, greeny,
   groping, headed, idiot, ignoramus, ignorant, illiterate,
   illiterati, inane, inexperienced, influenced, innocent, interested,
   jaundiced, know from nothing, lackwit, lowbrow, middlebrow, naive,
   nescient, no scholar, nonobjective, not know beans, one-sided,
   opinionated, partial, partisan, pinhead, prejudiced, prepossessed,
   puddinghead, racist, raw, rude, sexist, simple, strange to,
   superpatriotic, swayed, tenderfoot, tentative, twisted,
   ultranationalist, unacquainted, unapprized, uncomprehending,
   unconversant, undetached, undispassionate, uneducated,
   unenlightened, unfamiliar, unilluminated, uninformed, uninitiated,
   uninstructed, unintelligent, unintelligentsia, unknowing,
   unlettered, unposted, unripe, unsure, untaught, untutored,
   unversed, vacuous, wallow in ignorance, warped, xenophobic