The Collaborative International Dictionary of English v.0.48:

Byword \By"word`\, n. [AS. b["i]word; b["i], E. by + word.]
   1. A common saying; a proverb; a saying that has a general
      currency.
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            I knew a wise man that had it for a byword. --Bacon.
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   2. The object of a contemptuous saying.
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            Thou makest us a byword among the heathen. --Ps.
                                                  xliv. 14
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WordNet (r) 3.0 (2006):

byword
    n 1: a condensed but memorable saying embodying some important
         fact of experience that is taken as true by many people
         [syn: proverb, adage, saw, byword]