noun
water flowing underneath; hidden feelings
Literally water flowing beneath something, and figuratively one's true feelings hidden beneath the surface. The figurative use is somewhat literary.
橋の下を下水が静かに流れていた。
Water was quietly flowing underneath the bridge.
彼の言葉の下水には深い悲しみが感じられた。
Beneath his words, one could sense deep sadness.
noun
kanji 'water' radical at bottom (radical 85)
Refers to the 氺 (したみず) form of the water radical when it appears at the bottom of a kanji, as in 泰. This is a technical kanji term.
「泰」の字の下の部分は下水と呼ばれる部首です。
The bottom part of the character 泰 is the radical called したみず.