The Collaborative International Dictionary of English v.0.48:

Stifle \Sti"fle\, v. t. [imp. & p. p. Stifled; p. pr. & vb. n.
   Stifling.] [Freq. of OE. stif stiff; cf. Icel. st[imac]fla
   to dam up.]
   1. To stop the breath of by crowding something into the
      windpipe, or introducing an irrespirable substance into
      the lungs; to choke; to suffocate; to cause the death of
      by such means; as, to stifle one with smoke or dust.
      [1913 Webster]

            Stifled with kisses, a sweet death he dies.
                                                  --Dryden.
      [1913 Webster]

            I took my leave, being half stifled with the
            closeness of the room.                --Swift.
      [1913 Webster]

   2. To stop; to extinguish; to deaden; to quench; as, to
      stifle the breath; to stifle a fire or flame.
      [1913 Webster]

            Bodies . . . stifle in themselves the rays which
            they do not reflect or transmit.      --Sir I.
                                                  Newton.
      [1913 Webster]

   3. To suppress the manifestation or report of; to smother; to
      conceal from public knowledge; as, to stifle a story; to
      stifle passion.
      [1913 Webster]

            I desire only to have things fairly represented as
            they really are; no evidence smothered or stifled.
                                                  --Waterland.
      [1913 Webster]

WordNet (r) 3.0 (2006):

stifling
    adj 1: characterized by oppressive heat and humidity; "the
           summer was sultry and oppressive"; "the stifling
           atmosphere"; "the sulfurous atmosphere preceding a
           thunderstorm" [syn: sultry, stifling, sulfurous,
           sulphurous]
    n 1: forceful prevention; putting down by power or authority;
         "the suppression of heresy"; "the quelling of the
         rebellion"; "the stifling of all dissent" [syn:
         suppression, crushing, quelling, stifling]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "stifling":
   airless, breathless, breezeless, choking, close, constrictive,
   contrary, counterproductive, crosswise, extinguishment, hindering,
   hindersome, humid, ill-ventilated, in the way, inhibiting,
   inhibitive, interrupting, interruptive, muggy, obstructing,
   obstructive, obstruent, occlusive, oppressive, overpowering,
   quashing, quenching, repressive, restrictive, smothering, smudgy,
   squashing, squelching, sticky, strangling, stuffy, suffocating,
   sultry, suppression, suppressive, troublesome, unbearable,
   unendurable, unvented, unventilated, windless