The Collaborative International Dictionary of English v.0.48:

Sultry \Sul"try\ (s[u^]l"tr[y^]), a. [Compar. Sultrier
   (s[u^]l"tr[i^]*[~e]r); superl. Sultriest.] [From
   Sweltry.]
   1. Very hot, burning, and oppressive; as, Libya's sultry
      deserts.
      [1913 Webster]

            Such as, born beneath the burning sky
            And sultry sun, betwixt the tropics lie. --Dryden.
      [1913 Webster]

   2. Very hot and humid, or hot, close, stagnant, and
      oppressive, as air.
      [1913 Webster]

            When in the sultry glebe I faint,
            Or on the thirsty mountain plant.     --Addison.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sultry
    adj 1: sexually exciting or gratifying; "sensual excesses"; "a
           sultry look"; "a sultry dance" [syn: sensual, sultry]
    2: 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]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "sultry":
   Fescennine, Rabelaisian, airless, baking, bawdy, blue, breathless,
   broiling, burning, close, damp, dirty, erotic, filthy, foul,
   foul-mouthed, foul-spoken, foul-tongued, fulsome, hot, humid,
   impure, ithyphallic, lewd, livid, lurid, lustful, lusty, moist,
   mucky, muggy, nasty, obscene, offensive, oppressive, passionate,
   pornographic, provocative, raunchy, red-hot, ribald, salacious,
   scorching, scurrile, scurrilous, seductive, sensationalistic,
   sensual, sexy, sizzling, smoking-room, smothering, smudgy, smutty,
   soggy, steaming, steamy, sticky, stifling, stuffy, suffocating,
   sweltering, sweltry, tabloid, torrid, unchaste, unclean,
   unprintable, unrepeatable, vile, voluptuous