The Collaborative International Dictionary of English v.0.48:

Stagnant \Stag"nant\ (-nant), a. [L. stagnans, -antis, p. pr. of
   stagnare. See Stagnate.]
   1. That stagnates; not flowing; not running in a current or
      steam; motionless; hence, impure or foul from want of
      motion; as, a stagnant lake or pond; stagnant blood in the
      veins.
      [1913 Webster]

   2. Not active or brisk; dull; as, business is stagnant.
      [1913 Webster]

            That gloomy slumber of the stagnant soul. --Johnson.
      [1913 Webster]

            For him a stagnant life was not worth living.
                                                  --Palfrey.
      [1913 Webster]

WordNet (r) 3.0 (2006):

stagnant
    adj 1: not circulating or flowing; "dead air"; "dead water";
           "stagnant water" [syn: dead(a), stagnant]
    2: not growing or changing; without force or vitality [syn:
       stagnant, moribund]

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "stagnant":
   abeyant, apathetic, benumbed, blase, bored, cataleptic, catatonic,
   contaminated, contemplative, dead, debilitated, dirty, do-nothing,
   dopey, dormant, droopy, drugged, dull, enervated, exanimate,
   filthy, flat, foul, groggy, heavy, hebetudinous, idle, immobile,
   in abeyance, in suspense, inactive, inanimate, inert, jaded,
   lackadaisical, laissez-aller, laissez-faire, languid, languorous,
   latent, leaden, lethargic, lifeless, listless, logy, lumpish,
   meditative, moribund, motionless, neuter, neutral, numb, paralytic,
   paralyzed, passive, phlegmatic, polluted, pooped, procrastinating,
   putrefied, putrescent, putrid, quiescent, quiet, quietist,
   quietistic, sated, sedentary, slack, sleeping, sleepy, slow,
   sluggish, slumbering, smoldering, somnolent, stagnating, stale,
   standing, standpat, static, stationary, still, stultified, supine,
   suspended, tame, torpid, unaroused, unmoving, vegetable,
   vegetative, wan, weary, world-weary