noun
below the knee
Refers to the area or length below the knee. Often used in clothing descriptions (e.g., skirt length) or medical contexts.
このスカートは膝下までの丈です。
This skirt is knee-length (falls below the knee).
膝下を冷やさないようにしてください。
Please keep the area below your knees warm.
膝上 means 'above the knee', the opposite of 膝下.
Compound of 膝 (hiza, 'knee') and 下 (shita, 'below'). The reading ひざした is a straightforward combination.