The Collaborative International Dictionary of English v.0.48:

Swelter \Swel"ter\, v. i. [imp. & p. p. Sweltered; p. pr. &
   vb. n. Sweltering.] [From Swelt, v. i.]
   1. To be overcome and faint with heat; to be ready to perish
      with heat. "Sweltered cattle." --Coleridge.
      [1913 Webster]

   2. To welter; to soak. [Obs.] --Drayton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sweltering
    adj 1: excessively hot and humid or marked by sweating and
           faintness; "a sweltering room"; "sweltering athletes"
           [syn: sweltering, sweltry]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "sweltering":
   ardent, baking, blistering, boiling, broiling, burning,
   burning hot, canicular, clammy, close, ebullient, feverish, fiery,
   flushed, grilling, heated, hot, hot as fire, hot as hell, humid,
   like a furnace, like an oven, melting, muggy, oppressive,
   overheated, overwarm, parching, piping hot, red-hot, roasting,
   scalding, scorching, searing, seething, simmering, sizzling,
   sizzling hot, smoking hot, steaming, steamy, sticky, stifling,
   stuffy, sudorific, suffocating, sultry, sweating, sweaty, sweltry,
   toasting, torrid, tropical, wet, white-hot, wilting