adding hot water; topping up with hot water
Refers to the act of adding hot water to a bath or pot to adjust temperature or volume. Often used in contexts like bathhouses, cooking, or tea preparation.
The bath got lukewarm, so I added hot water.
Adjust the temperature to just right by adding hot water.
Compound of 差し (sashi, continuative form of 差す 'to pour, add') + 湯 (yu, 'hot water').