The Collaborative International Dictionary of English v.0.48:

Muggy \Mug"gy\ (m[u^]g"g[y^]), a. [Compar. Muggier
   (m[u^]g"g[i^]*[~e]r); superl. Muggiest.] [Cf. Icel. mugga
   mist, mugginess. Cf. 4th Mold.]
   1. Moist; damp; moldy; as, muggy straw.
      [1913 Webster]

   2. Warm, damp, and windless; uncomfortably hot and humid;
      sultry; as, muggy air, weather.
      [1913 Webster]