The Collaborative International Dictionary of English v.0.48:

Bumpkin \Bump"kin\, n. [The same word as bumkin, which Cotgrave
   defines thus: "Bumkin, Fr. chicambault, the luffe-block, a
   long and thick piece of wood, whereunto the fore-sayle and
   sprit-sayle are fastened, when a ship goes by the winde."
   Hence, a clumsy man may easily have been compared to such a
   block of wood; cf. OD. boomken a little tree. See Boom a
   pole.]
   An awkward, heavy country fellow; a clown; a country lout.
   "Bashful country bumpkins." --W. Irving.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "bumpkin":
   boor, bucolic, clod, clodhopper, clown, country bumpkin, farmer,
   gawk, gawky, hayseed, hick, hillbilly, looby, lout, lubber, lummox,
   oaf, provincial, rube, rustic, yokel