noun
waist (pants); pant rise
Refers to the vertical distance from the crotch seam to the top of the waistband on pants, trousers, or similar garments. A technical clothing term.
このズボンは股上が深いので、座っても腰が出ません。
These pants have a deep rise, so your lower back won't show even when you sit down.
股上の長さを測って、自分に合うサイズを選んでください。
Measure the rise length and choose a size that fits you.
股下 refers to the inseam length (from crotch to hem), while 股上 is the rise (crotch to waistband).
Compound of 股 (また, 'crotch') and 上 (うえ/がみ, 'above'). The reading がみ is a rendaku form of かみ (上).