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"