The Collaborative International Dictionary of English v.0.48:

Gusset \Gus"set\, n. [F. gousset armpit, fob, gusset, dim. of
   gousse pod, husk; cf. It. guscio shell, or W. cwysed gore,
   gusset.]
   1. A small piece of cloth inserted in a garment, for the
      purpose of strengthening some part or giving it a tapering
      enlargement.
      [1913 Webster]

            Seam and gusset and band.             --Hood.
      [1913 Webster]

   2. Anything resembling a gusset in a garment; as:
      (a) (Armor) A small piece of chain mail at the openings of
          the joints beneath the arms.
      (b) (Mach.) A kind of bracket, or angular piece of iron,
          fastened in the angles of a structure to give strength
          or stiffness; esp., the part joining the barrel and
          the fire box of a locomotive boiler.
          [1913 Webster]

   3. (Her.) An abatement or mark of dishonor in a coat of arms,
      resembling a gusset.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gusset
    n 1: a piece of chain mail covering a place unprotected by armor
         plate [syn: voider, gusset]
    2: a metal plate used to strengthen a joist [syn: gusset,
       gusset plate]
    3: a piece of material used to strengthen or enlarge a garment
       [syn: gusset, inset]