A gusset is a heraldic charge in the shape of a tierce, with a wedge taken out of the top section. A gusset sinister is considered an abatement indicating drunkenness.
A gusset is a small section of cloth, generally square, that is added to garment to allow a degree of movement. Typically they are found between an arm and the torso or between legs.
A large amount of early medieval clothing used this method in the construction of garments as it allowed a more economic use of fabric.