The Collaborative International Dictionary of English v.0.48:

Seignior \Seign"ior\, n. [OF. seignor, F. seigneur, cf. It.
   signore, Sp. se[~n]or from an objective case of L. senier
   elder. See Senior.]
   1. A lord; the lord of a manor.
      [1913 Webster]

   2. A title of honor or of address in the South of Europe,
      corresponding to Sir or Mr. in English.
      [1913 Webster]

   Grand Seignior, the sultan of Turkey.
      [1913 Webster]

WordNet (r) 3.0 (2006):

seignior
    n 1: a man of rank in the ancient regime [syn: feudal lord,
         seigneur, seignior]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

SEIGNIOR or SEIGNEUR. Among the feudists, this name signified lord of the
fee. F. N. B. 23. The most extended signification of this word includes not
only a lord or peer of parliament, but is applied to the owner or proprietor
of a thing; hence, the owner of a hawk, and the master of a fishing vessel,
is called a seigneur. 37 Edw. Ill. c. 19; Barr. on the Stat. 258.