The Collaborative International Dictionary of English v.0.48:

Sestet \Ses*tet"\, n. [It. sestetto, fr. sesto sixth, L. sextus,
   fr. sex six.]
   1. (Mus.) A piece of music composed for six voices or six
      instruments; a sextet; -- called also sestuor. [Written
      also sestett, sestette.]
      [1913 Webster]

   2. (Poet.) The last six lines of a sonnet.
      [1913 Webster]