noun
gusset
GUS-it/ˈɡʌsɪt/
A strengthening insert fitted between joined sections.
A gusset reinforces the seam where the sleeve meets the jacket.

Etymology
Old French gosset named an armhole or a piece of armor protecting the armpit. The term moved from armor to clothing, where it names an inserted fabric piece that strengthens or expands a garment.
In context
So this is where we're going to want to add the gusset piece or the piece to add us more fabric.
More examples
- The pants have a gusset that allows more room for movement.
- Steel gussets strengthen the joints between the roof beams.
- She sews a gusset into the bag to reinforce its bottom.
Forms
gusset · gussets
Similar words
- reinforcement
- Any added strengthening material, not necessarily an insert at a joint.
- brace
- A structural support, often a bar rather than an inserted plate.