減

A phono-semantic compound composed of:

水 (수, “water”) → gives the meaning (connection to liquid or the act of lessening/diluting);

咸 (함, “all, entire”) → provides the sound.

The idea is that adding water dilutes or lessens the intensity of something — leading to the extended sense of “to reduce.”

It appears in common compounds like:

減少 (감소) - decrease,

削減 (삭감) - reduction,

加減 (가감) - addition and subtraction.