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]

The Collaborative International Dictionary of English v.0.48:

Swelter \Swel"ter\, v. t.
   1. To oppress with heat. --Bentley.
      [1913 Webster]

   2. To exude, like sweat. [R.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

swelter
    v 1: be uncomfortably hot
    2: suffer from intense heat; "we were sweltering at the beach"

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "swelter":
   BO, THI, bake, be in heat, beaded brow, beads of sweat, blaze,
   bloom, body odor, boil, broil, burn, choke, closeness, cold sweat,
   combust, cook, diaphoresis, exudate, exudation, exude, flame,
   flame up, flare, flare up, flicker, flush, fry, gasp, glow,
   honest sweat, humidity, humidness, incandesce, lather, melt,
   mugginess, oppressiveness, pant, parch, perspiration,
   perspiration odor, perspire, radiate heat, roast, scald, scorch,
   seethe, shimmer with heat, simmer, smolder, smother, spark, steam,
   stew, stickiness, stifle, streams of sweat, stuffiness, sudor,
   sudoresis, suffocate, sweat, temperature-humidity index, toast,
   water, wilt