The Collaborative International Dictionary of English v.0.48:

Yokel \Yo"kel\, n. [Perhaps from an AS. word akin to E. gawk.]
   A country bumpkin. [Eng.] --Dickens.
   [1913 Webster]

WordNet (r) 3.0 (2006):

yokel
    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:

97 Moby Thesaurus words for "yokel":
   Babbitt, Philistine, arriviste, babe, backwoodsman, blockhead,
   blunderer, blunderhead, boor, boorish, botcher, bounder, bourgeois,
   bucolic, bumbler, bumpkin, bungler, cad, child, child of nature,
   churl, churlish, clod, cloddish, clodhopper, clodhopping,
   clodknocker, clot, clown, clownish, countrified, country bumpkin,
   country-born, country-bred, dolt, dove, dupe, epicier, farmer,
   farmerish, from the sticks, fumbler, gawk, gawky, gowk, groundling,
   guttersnipe, hayseed, hick, hicky, hillbilly, hobnailed, hooligan,
   ill-bred fellow, infant, ingenue, innocent, inurbane, klutz, lamb,
   looby, lout, loutish, low fellow, lubber, lummox, lumpen, lumpish,
   mere child, mucker, noble savage, nouveau riche, oaf, ox, parvenu,
   peasant, ribald, rough, roughneck, rowdy, rube, ruffian, rustic,
   simple soul, slouch, slubberer, uncouth, uncultivated, uncultured,
   unpolished, unrefined, unsophisticate, up-country, upstart,
   vulgarian, vulgarist, yokelish