leftover sake that has cooled
Refers specifically to sake that was warmed (燗) and then left to cool down. Often implies it is no longer at the ideal drinking temperature.
燗冷まし is warmed sake that has gone cold.
I warmed the sake before the guests arrived, but they were late so it ended up as cooled leftover sake.
Compound of 燗 (kan, 'warmed sake') + 冷まし (samashi, 'cooling down', nominalized form of 冷ます). The reading かんざまし results from rendaku voicing of さ to ざ.