What does "doublet" mean?

Additional senses

1. 2.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.
2. 3.(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.
3. 4.(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.
4. 5.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.
5. 6.pl. Etym: [Cf. Pr. doblier, dobler draughtboard.] A game somewhat like backgammon. Halliwell.
6. 7.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.