The Collaborative International Dictionary of English v.0.48:

Motto \Mot"to\, n.; pl. Mottoes. [It. motto a word, a saying,
   L. muttum a mutter, a grunt, cf. muttire, mutire, to mutter,
   mumble; prob. of imitative origin. Cf. Mot a word.]
   1. (Her.) A sentence, phrase, or word, forming part of an
      heraldic achievment.
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   2. A sentence, phrase, or word, prefixed to an essay,
      discourse, chapter, canto, or the like, suggestive of its
      subject matter; a short, suggestive expression of a
      guiding principle; a maxim.
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            It was the motto of a bishop eminent for his piety
            and good works, . . . "Serve God, and be cheerful."
                                                  --Addison.
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