The Collaborative International Dictionary of English v.0.48:

Lumpish \Lump"ish\, a.
   Like a lump; inert; gross; heavy; dull; spiritless. "
   Lumpish, heavy, melancholy." --Shak. -- Lump"ish*ly, adv.
   -- Lump"ish*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lumpish
    adj 1: mentally sluggish [syn: lumpish, lumpen,
           unthinking]

Moby Thesaurus II by Grady Ward, 1.0:

168 Moby Thesaurus words for "lumpish":
   Boeotian, all thumbs, apathetic, asinine, awkward, beef-brained,
   beef-witted, benumbed, blase, blockish, blunderheaded, blundering,
   boorish, bored, bovine, bulky, bumbling, bungling, burdensome,
   butterfingered, caked, cakey, careless, carlish, chumpish,
   churlish, clabbered, cloddish, clodhopping, clotted, clownish,
   clumsy, clumsy-fisted, coagulated, congealed, countrified,
   country-born, country-bred, cowish, crass, cumbersome, cumbrous,
   curded, curdled, dead, debilitated, dense, doltish, dopey, dormant,
   droopy, drugged, dull, dullard, dumb, duncical, duncish,
   elephantine, enervated, exanimate, farmerish, fat,
   fingers all thumbs, from the sticks, fumbling, gauche, gawkish,
   gawky, gelatinized, graceless, gross, grumous, ham-fisted,
   ham-handed, hayseed, heavy, heavy-handed, hebetudinous, hick,
   hicky, hobnailed, hooliganish, hulking, hulky, inanimate,
   incrassate, incumbent, ineducable, inelegant, inert, inspissate,
   inspissated, inurbane, jaded, jelled, jellied, klutzy,
   lackadaisical, languid, languorous, leaden, left-hand, left-handed,
   lethargic, lifeless, listless, loobyish, loutish, lowbred,
   lubberly, lumbering, lumpen, lumpy, maladroit, massive, massy,
   moribund, numb, oafish, onerous, opaque, oppressive, phlegmatic,
   ponderous, pooped, raffish, roughneck, rowdy, rowdyish, rube,
   ruffianly, sated, sleepy, sloppy, slow, sluggish, somnolent,
   sottish, splay, stagnant, stagnating, stiff, stultified, stupid,
   superincumbent, supine, thick, thickened, torpid, uncouth,
   uncultivated, uncultured, ungainly, ungraceful, unhandy,
   unpolished, unrefined, unteachable, unwieldy, up-country,
   vegetable, vegetative, wan, weary, world-weary, wrongheaded, yokel,
   yokelish