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]