Combine the water and sugar in a small saucepan and heat over low heat, stirring until the sugar dissolves completely. Let it cool to room temperature.
Pour the cooled sugar syrup into your blender, then add the chilled whiskey and water or juice. Blend briefly to combine the liquids.
Add the ice cubes to the blender, then blend on high until the mixture is smooth, icy, and slushy. You should see tiny ice crystals and a chilled texture.
Check the consistency; if it’s too runny, add a few more ice cubes and blend again until reaching an ice-crystal texture.
Pour the whiskey slushie into glasses, allowing the icy mixture to mound slightly above the rim for an inviting presentation.
Serve immediately with a straw or a garnish like a lemon twist or cherry, enjoying the frosty, aromatic experience.
