The Collaborative International Dictionary of English v.0.48:

Weightless \Weight"less\, a.
   Having no weight; imponderable; hence, light. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

weightless
    adj 1: having little or no weight or apparent gravitational
           pull; light; "floating freely in a weightless condition";
           "a baby bat...fluffy and weightless as a moth"; "jackets
           made of a weightless polyester fabric" [ant: weighty]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "weightless":
   airy, bodiless, bubbly, downy, ethereal, featherweight, feathery,
   fluffy, foamy, frothy, gossamery, immaterial, impalpable,
   imponderable, imponderous, incorporeal, insubstantial, intangible,
   leger, light, light as air, lighter than vanity, lightweight,
   mousse, nonsubstantial, souffle, unconcrete, unheavy, unsolid,
   unsubstanced, unsubstantial, volatile, yeasty