水割り (mizuwari) means “diluted alcoholic drink”

More examples

食事の前に水割りを一杯いただきます。shoku ji no mae ni mizu wa ri o ip pai i ta da ki ma su

I have one whiskey with water before a meal.

Describes liquor whose strength is softened by mixing it with water rather than drinking it straight.

お湯割りoyuwari: A drink made by mixing liquor with hot water.
ハイボールhaibōru: A drink made by mixing whisky with carbonated water.

酒に水を加えて飲みやすくした飲み物。sake ni mizu o kuwa e te no mi ya su ku shi ta no mi mono

水割り (mizuwari) combines 水 (mizu, "water") and 割り (wari, "mixing or dividing") to name an alcoholic drink mixed with water.

In Japanese bars, 水割り (mizuwari) commonly means whisky mixed with still water, whereas a highball is whisky mixed with carbonated water.