冷酷 is a common word for 'cruelty' or 'cold-heartedness'. 冷酷無残 is a stronger, more emphatic yojijukugo that adds the nuance of being utterly merciless and without pity.
無残 alone means 'cruel' or 'merciless', but 冷酷無残 combines it with 冷酷 to intensify the meaning into a set four-character idiom.
Etymology
A yojijukugo combining 冷酷 (cold-hearted cruelty) and 無残 (mercilessness). The exact historical origin is uncertain, but it follows the typical pattern of intensifying compounds in classical Chinese-derived idioms.
