闸
zhá · HSK 7
sluice
🚧 门 gate + 甲 = a gate inside a gate that halts flow: sluice, lock, or brake
The 门 radical wraps the whole character, so 闸 is literally a gate set into a doorway. A water-gate holds back a river (sluice/lock); the same hold-it-back idea gives the meaning brake.
闸 — gate radical 門 + 甲 (jiǎ, phonetic)
闸 is a phono-semantic compound: 門 'gate' gives the meaning, while 甲 hints at the sound. It originally referred to a sluice gate that could be raised or lowered to control water flow, and later extended to mechanical brakes and electrical switches that similarly stop or release flow.