The Collaborative International Dictionary of English v.0.48:

Signor \Si*gnor"\, Signore \Si*gno"re\, n. [It. See Seignior.]
   Sir; Mr.; -- a title of address or respect among the
   Italians. Before a noun the form is Signor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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