The Collaborative International Dictionary of English v.0.48:

Gunny \Gun"ny\ (g[u^]n"n[y^]), n., Gunny cloth \Gun"ny
cloth`\(kl[o^]th`; 115). [Hind. go[.n], go[.n][imac], a sack,
   sacking.]
   A strong, coarse kind of sacking, made from the fibers
   (called jute) of two plants of the genus Corchorus (C.
   olitorius and C. capsularis), of India. The fiber is also
   used in the manufacture of cordage.
   [1913 Webster]

   Gunny bag or Gunny sack, a sack made of gunny or burlap,
      used for coarse commodities. In the southern U. S. similar
      sacks are called crocus sack, croker sack, towsack,
      and grass sack.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

gunny sack
    n 1: a bag made of burlap [syn: gunnysack, gunny sack,
         burlap bag]