The Collaborative International Dictionary of English v.0.48:

Stolid \Stol"id\, a. [L. stolidus.]
   Hopelessly insensible or stupid; not easily aroused or
   excited; dull; impassive; foolish.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stolid
    adj 1: having or revealing little emotion or sensibility; not
           easily aroused or excited; "her impassive remoteness";
           "he remained impassive, showing neither interest in nor
           concern for our plight"- Nordhoff & Hall; "a silent
           stolid creature who took it all as a matter of
           course"-Virginia Woolf; "her face showed nothing but
           stolid indifference" [syn: impassive, stolid]

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "stolid":
   aloof, apathetic, arid, at rest, barren, blunt, bored, bovine,
   calm, careless, cloistered, cool, dense, detached, disinterested,
   dispassionate, distant, doltish, dry, dull, dumb, dwindling,
   earthbound, ebbing, even-tempered, even-tenored, fact, halcyon,
   heedless, hushed, impassive, imperturbable, inactive, incurious,
   indifferent, inert, inexcitable, infecund, infertile, inirritable,
   insouciant, isolated, lethargic, listless, literal, lumpish,
   mindless, moldering, mundane, obtuse, pacific, passive, patient,
   peaceable, peaceful, phlegmatic, placid, prosaic, prosing, prosy,
   quiescent, quiet, regardless, reposeful, reposing, restful,
   resting, secluded, sequestered, sequestrated, sheltered, slow,
   smooth, staid, steady, still, still as death, stillish, stilly,
   stoic, stuffy, stupid, subsiding, supine, thick, tranquil,
   unagitated, unconcerned, undisturbable, undisturbed, unemotional,
   unfanciful, unflappable, unideal, unimaginative, uninquiring,
   uninspired, uninterested, uninventive, uninvolved, unirritable,
   unmindful, unmoved, unnervous, unoriginal, unpassionate,
   unperturbed, unpoetic, unromantic, unromanticized, unruffled,
   unstirring, untroubled, waning, withdrawn, wooden