The Collaborative International Dictionary of English v.0.48:

Usbegs \Us"begs\, Usbeks \Us"beks\, n. pl. (Ethnol.)
   A Turkish tribe which about the close of the 15th century
   conquered, and settled in, that part of Asia now called
   Turkestan. [Written also Uzbecks, and Uzbeks.]
   [1913 Webster]