The Collaborative International Dictionary of English v.0.48:

G-string \G-string\ n.
   1. a patch of cloth attached to and supported by string-like
      cords, worn over the hips, and serving to cover only the
      pubic area; -- often worn by stripteasers.

   Syn: thong, merkin.
        [WordNet 1.5]

   2. A small covering for the loins.

   Syn: breechcloth, breechclout, loincloth.
        [WordNet 1.5]

   3. [from Dr. Graubau.] (Physics) A transmission line
      consisting a single wire coated with dielectric, thus
      permitting surface-wave propagation. [slang] --Dict. Sci.
      Technol.
      [PJC] G suit

WordNet (r) 3.0 (2006):

G-string
    n 1: minimal clothing worn by stripteasers; a narrow strip of
         fabric that covers the pubic area, passes between the
         thighs, and is supported by a waistband [syn: G-string,
         thong]