The Collaborative International Dictionary of English v.0.48:

Signior \Sign"ior\, n.
   Sir; Mr. The English form and pronunciation for the Italian
   Signor and the Spanish Se[~n]or.
   [1913 Webster]

WordNet (r) 3.0 (2006):

signior
    n 1: used as an Italian courtesy title; can be prefixed to the
         name or used separately [syn: signor, signior]