The Collaborative International Dictionary of English v.0.48:

Dey \Dey\, n.; pl. Deys. [Turk. d[=a]i, orig., a maternal
   uncle, then a friendly title formerly given to middle-aged or
   old people, especially among the Janizaries; and hence, in
   Algiers, consecrated at length to the commanding officer of
   that corps, who frequently became afterward pasha or regent
   of that province; hence the European misnomer of dey, as
   applied to the latter: cf. F. dey.]
   The governor of Algiers; -- so called before the French
   conquest in 1830.
   [1913 Webster]