noun
refilling; topping up
Refers to the act of adding more liquid to a container that already has some, such as topping up a drink or refilling a pot.
お茶の注ぎ足しをお願いします。
Please top up the tea.
鍋に水を注ぎ足しして、もう少し煮込もう。
Let's add more water to the pot and simmer it a bit longer.
おかわり is a general term for a second helping (often food or drink), while 注ぎ足し specifically means refilling a liquid into the same container.
Compound of 注ぎ (continuative form of 注ぐ 'to pour') + 足し (continuative form of 足す 'to add').