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]