noun
inner pleat; inner tuck; fabric raised inside to adjust length
A tailoring technique used in kimono and yukata: excess fabric is folded up and stitched inside to shorten the garment while keeping the outer appearance neat. The term is domain-specific to traditional Japanese clothing.
着物の内揚げを直してもらった。
I had the inner tuck of my kimono adjusted.
子供の浴衣は内揚げをして長さを調節する。
For a child's yukata, you adjust the length by making an inner tuck.