The Collaborative International Dictionary of English v.0.48:

Phlegm \Phlegm\ (fl[e^]m), n. [F. phlegme, flegme, L. phlegma,
   fr. Gr. fle`gma a flame, inflammation, phlegm, a morbid,
   clammy humor in the body, fr. fle`gein to burn. Cf. Phlox,
   Flagrant, Flame, Bleak, a., and Fluminate.]
   1. One of the four humors of which the ancients supposed the
      blood to be composed. See Humor. --Arbuthnot.
      [1913 Webster]

   2. (Physiol.) Viscid mucus secreted in abnormal quantity in
      the respiratory and digestive passages.
      [1913 Webster]

   3. (Old Chem.) A watery distilled liquor, in distinction from
      a spirituous liquor. --Crabb.
      [1913 Webster]

   4. Sluggishness of temperament; dullness; want of interest;
      indifference; coldness.
      [1913 Webster]

            They judge with fury, but they write with phlegm.
                                                  --Pope.
      [1913 Webster]

WordNet (r) 3.0 (2006):

phlegm
    n 1: apathy demonstrated by an absence of emotional reactions
         [syn: emotionlessness, impassivity, impassiveness,
         phlegm, indifference, stolidity, unemotionality]
    2: expectorated matter; saliva mixed with discharges from the
       respiratory passages; in ancient and medieval physiology it
       was believed to cause sluggishness [syn: phlegm, sputum]
    3: inactivity; showing an unusual lack of energy; "the general
       appearance of sluggishness alarmed his friends" [syn:
       languor, lethargy, sluggishness, phlegm, flatness]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "phlegm":
   acedia, aloofness, apathy, ataraxia, ataraxy, benumbedness, blah,
   blahs, boredom, calmness, chyle, colostrum, comatoseness,
   composure, coolness, detachment, discharge, disinterest,
   dispassion, drowsiness, dullness, enervation, ennui, fatigue,
   gleet, heartlessness, heaviness, hebetude, hopelessness, humor,
   ichor, impassivity, imperturbability, inanimation, inappetence,
   indifference, insensibility, insouciance, jadedness, lachryma,
   lack of appetite, lackadaisicalness, lactation, languidness,
   languishment, languor, languorousness, lassitude, lenitude, lentor,
   lethargicalness, lethargy, leukorrhea, lifelessness, listlessness,
   lymph, matter, milk, mucor, mucus, nonchalance, numbness,
   oscitancy, passiveness, passivity, peccant humor, phlegmaticalness,
   phlegmaticness, plucklessness, purulence, pus, resignation,
   resignedness, rheum, saliva, sangfroid, sanies, satedness,
   self-possession, serous fluid, serum, sleepiness, sloth,
   slothfulness, slowness, sluggishness, snot, somnolence, sopor,
   soporifousness, spiritlessness, spunklessness, stoicism, stolidity,
   stupefaction, stupor, supineness, suppuration, sweat, tear,
   teardrop, the whites, torpidity, torpidness, torpitude, torpor,
   unconcern, unresponsiveness, urine, weariness, withdrawnness,
   world-weariness