减速
jiǎn sù · HSK 7
to slow down
🐢 减速 = 减 reduce + 速 speed
减 means subtract or cut down; 速 means speed or velocity. Stack them and you literally get cut-down-speed, the act of slowing a car or pace.
减 — water 氵 + 咸 (xián, all/salty)
A semantic-phonetic compound. The water radical 氵 gives the meaning, originally suggesting water decreasing or draining away, while 咸 supplies the sound (jiǎn shifted from xián). From 'water lessening' the sense broadened to 'reduce, subtract, diminish' in general use.