The Collaborative International Dictionary of English v.0.48:

Sheik \Sheik\, n. [Ar. sheikh, shaykh, a venerable old man, a
   chief, fr. sh[=a]kha to grow or be old.]
   The head of an Arab family, or of a clan or a tribe; also,
   the chief magistrate of an Arab village. The name is also
   applied to Mohammedan ecclesiastics of a high grade. [Written
   also scheik, shaik, sheikh.]
   [1913 Webster] Sheil

WordNet (r) 3.0 (2006):

sheikh
    n 1: the leader of an Arab village or family [syn: sheik,
         tribal sheik, sheikh, tribal sheikh, Arab chief]