WordWeb Online

1. Lacking in sophistication or worldliness
   "his ingenuous explanation that he would not have burned the church if he had not thought the bishop was in it";
   - innocent
    
2. Characterized by an inability to mask your feelings; not devious
   "an ingenuous admission of responsibility";
   - artless

See also: candid, heart-to-heart, naif, naïf, naive, naïve, open, sincere, undistorted, wet behind the ears