Source : Webster's Revised Unabridged Dictionary (1913)

Signior \Sign"ior\, n.
   Sir; Mr. The English form and pronunciation for the Italian
   Signor and the Spanish Se[~n]or.

Source : WordNet®

signior
     n : used as an Italian courtesy title; can be prefixed to the
         name or used separately [syn: {signor}]
     [also: {signori} (pl)]