The Collaborative International Dictionary of English v.0.48:

hayseed \hay"seed\ n.
   1. a rural, unsophisticated person; also used in an extended
      sense for one who is not very intelligent or uninterested
      in culture.

   Syn: yokel, rube, hick, yahoo, bumpkin, chawbacon.
        [WordNet 1.5]

   2. Seed from grass, especially that which falls out of hay.
      [PJC]

WordNet (r) 3.0 (2006):

hayseed
    n 1: a person who is not very intelligent or interested in
         culture [syn: yokel, rube, hick, yahoo, hayseed,
         bumpkin, chawbacon]