The Collaborative International Dictionary of English v.0.48:

Phlegmatic \Phleg*mat"ic\, a. [L. phlegmaticus, Gr. ?: cf. F.
   phlegmatique.]
   1. Watery. [Obs.] "Aqueous and phlegmatic." --Sir I. Newton.
      [1913 Webster]

   2. Abounding in phlegm; as, phlegmatic humors; a phlegmatic
      constitution. --Harvey.
      [1913 Webster]

   3. Generating or causing phlegm. "Cold and phlegmatic
      habitations." --Sir T. Browne.
      [1913 Webster]

   4. Not easily excited to action or passion; cold; dull;
      sluggish; heavy; as, a phlegmatic person. --Addison.
      [1913 Webster]

   Phlegmatic temperament (Old Physiol.), lymphatic
      temperament. See under Lymphatic.
      [1913 Webster]

WordNet (r) 3.0 (2006):

phlegmatic
    adj 1: showing little emotion; "a phlegmatic...and certainly
           undemonstrative man" [syn: phlegmatic, phlegmatical]

Moby Thesaurus II by Grady Ward, 1.0:

152 Moby Thesaurus words for "phlegmatic":
   Laodicean, Olympian, abeyant, aloof, ambivert, apathetic, benumbed,
   blah, blase, bored, calm, careless, cataleptic, catatonic,
   choleric, cold, comatose, composed, controlled, cool, cycloid,
   cyclothyme, cyclothymic, dead, debilitated, desensitized, detached,
   disinterested, distant, dopey, dormant, droopy, drugged, dry, dull,
   ectomorph, endomorph, enervated, equanimous, even-tempered,
   exanimate, extrovert, flat, foul, groggy, heartless, heavy,
   hebetudinous, heedless, hopeless, impassive, imperturbable,
   in a stupor, in abeyance, in suspense, inactive, inanimate,
   incurious, indifferent, indolent, inert, insensible, insensitive,
   insouciant, introvert, jaded, lackadaisical, languid, languorous,
   latent, leaden, lethargic, lifeless, listless, logy, lumpish,
   matter-of-fact, melancholic, mesomorph, mindless, moderate,
   moribund, nonchalant, numb, numbed, passive, philosophical, placid,
   pluckless, pooped, regardless, resigned, restrained, sanguine,
   sated, schizoid, schizothyme, sedentary, self-controlled,
   self-possessed, serene, slack, sleeping, sleepy, slow, sluggish,
   slumbering, smoldering, somnolent, soporific, spiritless,
   spunkless, stagnant, stagnating, standing, static, stoic, stoical,
   stolid, stultified, stupefied, supine, suspended, syntone, tame,
   temperate, torpid, tranquil, unaffected, unanimated, unaroused,
   uncaring, unconcerned, undemonstrative, undisturbed, unemotional,
   unenthusiastic, unfeeling, uninquiring, uninterested, uninvolved,
   unmindful, unmoved, unperturbed, unresponsive, unruffled,
   vegetable, vegetative, wan, weary, withdrawn, world-weary