Source : Webster's Revised Unabridged Dictionary (1913)

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.

            Seam and gusset and band.             --Hood.

   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.

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

Source : WordNet®

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