The Collaborative International Dictionary of English v.0.48:

lumpen \lump"en\ adj.
   1. mentally sluggish.

   Syn: lumpish, sluggish, unthinking.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Lumpen \Lump"en\, n.
   1. (Politics) same as lumpenproletariat.
      [PJC]

   2. (Zool.) The European eelpout; -- called also lumper.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lumper \Lump"er\, n. [Cf. Lamper eel.] (Zool.)
   The European eelpout; -- called also lumpen.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

166 Moby Thesaurus words for "lumpen":
   abject, abominable, amorphic, amorphous, anarchic, arrant,
   at leisure, at liberty, at loose ends, atrocious, available, baggy,
   base, beat-up, bedraggled, beggarly, blobby, blowzy, blurred,
   blurry, boorish, careless, chaotic, characterless, cheesy, chintzy,
   churlish, cloddish, clodhopping, clownish, confused, contemptible,
   countrified, country-born, country-bred, crummy, debased, degraded,
   depraved, despicable, dilapidated, dirty, disengaged, disgusting,
   disorderly, drabbletailed, draggled, draggletailed, execrable,
   fallow, farmerish, featureless, flagrant, formless, foul, free,
   from the sticks, frowzy, frumpish, frumpy, fulsome, fuzzy, grave,
   gross, grubby, hayseed, hazy, heinous, hick, hicky, hobnailed,
   idle, in rags, inchoate, indecisive, indefinite, indeterminate,
   inform, informal, inurbane, jobless, kaleidoscopic, leisure,
   leisured, little, loose, loutish, low, low-down, lumpish, mangy,
   mean, measly, messy, miserable, misty, monstrous, mussy, nefarious,
   negligent, nondescript, obnoxious, obscure, odious, off, off duty,
   off work, orderless, otiose, out of employ, out of harness,
   out of work, paltry, petty, poky, poor, ragged, raggedy, rank,
   reptilian, rube, ruinous, scabby, scraggly, scrubby, scruffy,
   scummy, scurvy, seedy, shabby, shapeless, shoddy, slack,
   slatternly, slipshod, sloppy, slovenly, sluttish, small, sordid,
   squalid, tacky, tattered, unclear, uncouth, uncultivated,
   uncultured, undefined, unemployable, unemployed, unkempt,
   unmentionable, unneat, unoccupied, unordered, unorganized,
   unpolished, unrefined, unsightly, untidy, up-country, vague, vile,
   wretched, yokel, yokelish