The Collaborative International Dictionary of English v.0.48:

Naif \Na"["i]f`\ (?; formerly ?), a. [F. na["i]f. See
   Na["i]ve.]
   1. Having a true natural luster without being cut; -- applied
      by jewelers to a precious stone.
      [1913 Webster]

   2. Na["i]ve; as, a na["i]f remark. --London Spectator.
      [1913 Webster]

WordNet (r) 3.0 (2006):

naif
    adj 1: marked by or showing unaffected simplicity and lack of
           guile or worldly experience; "a teenager's naive
           ignorance of life"; "the naive assumption that things can
           only get better"; "this naive simple creature with wide
           friendly eyes so eager to believe appearances" [syn:
           naive, naif] [ant: sophisticated]
    n 1: a naive or inexperienced person

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "naif":
   babe, boob, child, child of nature, chump, cinch, credulous person,
   cull, dove, dupe, easy mark, easy pickings, fall guy, fish, fool,
   gobe-mouches, greener, greenhorn, greeny, gudgeon, gull, hick,
   infant, ingenue, innocent, lamb, leadpipe cinch, lout, mere child,
   monkey, noble savage, oaf, patsy, pigeon, plaything, prize sap,
   pushover, rube, sap, saphead, schlemiel, simple soul, sitting duck,
   stooge, sucker, toy, trusting soul, unsophisticate, victim,
   yokel