The Collaborative International Dictionary of English v.0.48:

Impi \Im"pi\ ([i^]m"p[i^]), n. [Zulu.]
   A body of Kaffir warriors; a body of native armed men. [South
   Africa]
   [Webster 1913 Suppl.]

         As early as 1862 he crossed assagais with and defeated
         a Matabili impi (war band).              --James Bryce.
   [Webster 1913 Suppl.]