Gusset

Chamber's Twientieth Century Dictionary · 1908 · p. 46

gus′et, n. the piece of cloth in a shirt which covers the armpit: an angular piece of cloth inserted in a garment to strengthen some part of it.— v.t. to make with a gu et: to insert a gu et into. [O. Fr. gou et — gou e —It. guscio , a pod, husk.]