The Collaborative International Dictionary of English v.0.48:

doltish \dolt"ish\ (d[=o]lt"[i^]sh), a.
   Doltlike; dull in intellect; stupid; blockish; as, a doltish
   clown. -- Dolt"ish*ly, adv. -- Dolt"ish*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

doltish
    adj 1: heavy and dull and stupid [syn: cloddish, doltish]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "doltish":
   Boeotian, asinine, beef-brained, beef-witted, beetleheaded,
   blockheaded, blockish, bovine, chumpish, cloddish, cowish, crass,
   dense, dull, dullard, dumb, duncical, duncish, fat, fatheaded,
   gross, ineducable, klutzy, lumpish, oafish, opaque, sottish,
   stupid, thick, unteachable, wrongheaded