The Collaborative International Dictionary of English v.0.48:

Ingenuous \In*gen"u*ous\, a. [L. ingenuus inborn, innate,
   freeborn, noble, frank; pref. in- in + the root of gignere to
   beget. See Genius, and cf. Ingenious.]
   1. Of honorable extraction; freeborn; noble; as, ingenuous
      blood of birth.
      [1913 Webster]

   2. Noble; generous; magnanimous; honorable; upright;
      high-minded; as, an ingenuous ardor or zeal.
      [1913 Webster]

            If an ingenuous detestation of falsehood be but
            carefully and early instilled, that is the true and
            genuine method to obviate dishonesty. --Locke.
      [1913 Webster]

   3. Free from reserve, disguise, equivocation, or
      dissimulation; open; frank; as, an ingenuous man; an
      ingenuous declaration, confession, etc.
      [1913 Webster]

            Sensible in myself . . . what a burden it is for me,
            who would be ingenuous, to be loaded with courtesies
            which he hath not the least hope to requite or
            deserve.                              --Fuller.
      [1913 Webster]

   4. Ingenious. [Obs.] --Shak.
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   Note: (Formerly) printers did not discriminate between . . .
         ingenuous and ingenious, and these words were used or
         rather printed interchangeably almost to the beginning
         of the eighteenth century. --G. P. Marsh.

   Syn: Open; frank; unreserved; artless; plain; sincere;
        candid; fair; noble; generous.

   Usage: Ingenuous, Open, Frank. One who is open speaks
          out at once what is uppermost in his mind; one who is
          frank does it from a natural boldness, or dislike of
          self-restraint; one who is ingenuous is actuated by a
          native simplicity and artlessness, which make him
          willing to confess faults, and make known his
          sentiments without reserve. See Candid.
          [1913 Webster]

WordNet (r) 3.0 (2006):

ingenuous
    adj 1: characterized by an inability to mask your feelings; not
           devious; "an ingenuous admission of responsibility" [syn:
           ingenuous, artless] [ant: artful, disingenuous]
    2: lacking in sophistication or worldliness; "a child's innocent
       stare"; "his ingenuous explanation that he would not have
       burned the church if he had not thought the bishop was in it"
       [syn: innocent, ingenuous]

Moby Thesaurus II by Grady Ward, 1.0:

115 Moby Thesaurus words for "ingenuous":
   artless, bluff, blunt, bold, born yesterday, broad, brusque,
   budding, callow, candid, childlike, confiding, cullible,
   deceivable, deludable, dewy, direct, downright, dupable, easy,
   explicit, exploitable, fair, fair and square, foolable, forthright,
   frank, frankhearted, free, free-speaking, free-spoken,
   free-tongued, genuine, green, growing, guileless, gullible,
   heart-to-heart, hoaxable, honest, honorable, hoodwinkable,
   humbugable, immature, impubic, inexperienced, ingenu, innocent,
   intact, juicy, just, minor, naive, natural, new-fledged,
   on the level, open, openhearted, outspoken, persuadable, plain,
   plain-spoken, raw, ripening, round, sappy, seduceable, simple,
   simplehearted, simpleminded, sincere, single-hearted,
   single-minded, soft, straight, straight-out, straightforward,
   suggestible, tender, transparent, trustful, trusting, trustworthy,
   unabashed, unadult, unaffected, unartificial, unchecked,
   uncomplicated, unconstrained, undeceitful, undeceptive, underage,
   undeveloped, undissembling, unequivocal, unfeigning, unfledged,
   unformed, unguarded, uninhibited, unlicked, unmellowed, unreserved,
   unrestrained, unripe, unschooled, unseasoned, unsophisticated,
   unstudied, unsuspicious, unwary, vernal, victimizable, virginal