The Collaborative International Dictionary of English v.0.48:

Pious \Pi"ous\, a. [L. pius: cf. F. pieux.]
   1. Of or pertaining to piety; exhibiting piety; reverential;
      dutiful; religious; devout; godly. "Pious hearts."
      --Milton. "Pious poetry." --Johnson.
      [1913 Webster]

            Where was the martial brother's pious care? --Pope.
      [1913 Webster]

   2. Practiced under the pretext of religion; prompted by
      mistaken piety; as, pious errors; pious frauds.
      [1913 Webster]

   Syn: Godly; devout; religious; righteous.
        [1913 Webster]

WordNet (r) 3.0 (2006):

pious
    adj 1: having or showing or expressing reverence for a deity;
           "pious readings" [ant: impious]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "pious":
   Christian, Christianlike, Christianly, God-fearing, Tartuffian,
   Tartuffish, adoring, affected, angelic, assured, believing,
   canting, certain, confident, convinced, cultish, cultist,
   cultistic, dedicated, devoted, devout, dogmatic, doubtless,
   dutiful, faithful, false, fideistic, fraudulent, godlike, godly,
   good, goody, goody-goody, holier-than-thou, holy, hypocritical,
   impressed with, insincere, mealy-mouthed, mealymouthed, moral,
   oily, persuaded, pharisaic, pietistic, pistic, positive, prayerful,
   pretended, priestly, religious, reverent, reverential,
   right-minded, saintly, sanctified, sanctimonious, satisfied,
   secure, self-righteous, seraphic, smarmy, sniveling, sold on,
   solemn, spiritual, sure, theistic, unctuous, under the impression,
   undoubtful, undoubting, venerational, venerative, virtuous,
   worshipful