The Collaborative International Dictionary of English v.0.48:

Kamsin \Kam*sin"\, Khamsin \Kham*sin"\, n. [Ar. khams[imac]n,
   fr. khams[=u]n, oblique case khams[imac]n, fifty; -- so
   called because it blows for about fifty days, from April till
   June.]
   A hot southwesterly wind in Egypt, coming from the Sahara.
   [Written also Khamseen.]
   [1913 Webster]