Source : Webster's Revised Unabridged Dictionary (1913)

Doublet \Doub"let\, n. [In sense 3, OF. doublet; in sense 4, F.
   doublet, dim. of double double. See {Double}, a.]
   1. Two of the same kind; a pair; a couple.

   2. (Print.) A word or words unintentionally doubled or set up
      a second time.

   3. A close-fitting garment for men, covering the body from
      the neck to the waist or a little below. It was worn in
      Western Europe from the 15th to the 17th century.

   4. (Lapidary Work) A counterfeit gem, composed of two pieces
      of crystal, with a color them, and thus giving the
      appearance of a naturally colored gem. Also, a piece of
      paste or glass covered by a veneer of real stone.

   5. (Opt.) An arrangement of two lenses for a microscope,
      designed to correct spherical aberration and chromatic
      dispersion, thus rendering the image of an object more
      clear and distinct. --W. H. Wollaston.

   6. pl. (See No. 1.) Two dice, each of which, when thrown, has
      the same number of spots on the face lying uppermost; as,
      to throw doublets.

   7. pl. [Cf. Pr. doblier, dobler draughtboard.] A game
      somewhat like backgammon. --Halliwell.

   8. One of two or more words in the same language derived by
      different courses from the same original from; as, crypt
      and grot are doublets; also, guard and ward; yard and
      garden; abridge and abbreviate, etc.

Source : WordNet®

doublet
     n : a man's close-fitting jacket; worn during the Renaissance