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to pour; to add (liquid); to serve (drinks)
to put on (lipstick, etc.); to apply; to colour; to dye
お茶にお湯をさして薄めてください。
Please add hot water to the tea to dilute it.
花瓶に水をさすのを忘れていた。
I forgot to add water to the vase.
杯にお酒をさすのは相手の役目だ。
Pouring sake into the cup is the other person s role.