Douglas Harper's Etymology Dictionary
1590s, from un- (1) "not" + compassionate (adj.).
Wiktionary
a. Not compassionate.
WordNet
adj. lacking compassion or feeling for others; "nor silver-shedding tears could penetrate her uncompassionate sire"- Shakespeare [ant: compassionate]
Usage examples of "uncompassionate".
I had seen what these things, half animal, used their genitals for, beyond the true purpose, and I was arrogant still with a raw and uncompassionate arrogance.
Yet from childhood on, Peter had been cruel and uncompassionate, and Andrew and Valentine feared him.
Arbuton, and looked about him with an eye of cold, uncompassionate inspection, as if he were trying it by a standard of taste, and, on the whole, finding the poor little church vulgar.
They had no taste for what they handled, from either a nationalistic or medical viewpoint, and not too much compassion, being hardened men of war fighting a singularly uncompassionate foe.