The Collaborative International Dictionary of English v.0.48:

Yataghan \Yat"a*ghan\, n. [Turk. y[=a]t[=a]gh[=a]n.]
   A long knife, or short saber, common among Mohammedan
   nations, usually having a double curve, sometimes nearly
   straight. [Written also ataghan, attaghan.] --Chaucer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

yataghan
    n 1: a long Turkish knife with a curved blade having a single
         edge