The Collaborative International Dictionary of English v.0.48:

Murky \Murk"y\, a. [Compar. Murkier; superl. Murkiest.] [OE.
   mirke, merke, AS. myrce, mirce; akin to Icel. myrkr, Dan. &
   Sw. m["o]rk.]
   1. Dark; gloomy. "The murkiest den." --Shak.
      [1913 Webster]

            A murky deep lowering o'er our heads. --Addison.
      [1913 Webster]

   2. Obscured by haze or mist; clouded; turbid; as, poor
      visibility in the murky water.
      [PJC]

WordNet (r) 3.0 (2006):

murky
    adj 1: (of liquids) clouded as with sediment; "a cloudy liquid";
           "muddy coffee"; "murky waters" [syn: cloudy, muddy,
           mirky, murky, turbid]
    2: dark or gloomy; "a murky dungeon"; "murky rooms lit by smoke-
       blackened lamps" [syn: murky, mirky]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "murky":
   ambiguous, amorphous, amphibological, besmirched, black, blackened,
   bleak, blind, blotchy, caliginous, clear as mud, clouded, cloudy,
   dark, dark-colored, darkened, darkish, darksome, dim, dimmish,
   dimpsy, dingy, dirty, discolored, dismal, double-edged,
   double-faced, drab, dreary, dun, dusk, dusky, equivocal, filthy,
   foggy, foul, foxed, foxy, fuliginous, funereal, fuzzy, glooming,
   gloomy, glowering, grey, grim, grimy, grubby, hazy, indeterminate,
   indistinct, inky, lowering, misty, mucky, muddy, murk, murksome,
   nasty, nebulous, nubilous, obscure, opaque, overcast, roily,
   semidark, shadowy, shady, shapeless, sibylline, smirched, smoky,
   smudgy, smutty, soiled, somber, sooty, sordid, squalid, stained,
   stigmatic, stigmatiferous, stigmatized, subfusc, tainted,
   tarnished, tenebrous, threatening, transcendent, turbid, unclean,
   unclear, unplain, vague