The characters literally combine to mean “in the heart, cannot endure/bear it.” The feeling of compassion originates deep inside (于心) and results in an inability to be ruthless (不忍).
The idiom 于心不忍 is deeply connected to the core Confucian value of 仁 (rén), which translates to benevolence, humaneness, or compassion. The famous philosopher Mencius argued that all humans are born with a `恻隐之心 (cèyǐn zhī xīn)`, a heart of compassion, which is the very seed of `仁`. When you see a child about to fall into a well, you instinctively feel alarm and distress—that is `恻隐之心`. 于心不忍 is the practical, everyday expression of this philosophical concept. It's the moment when your innate `仁` overrides a logical, harsh, or necessary action. Comparison to a Western Concept: Compare it to having a “pang of conscience” or being “soft-hearted.” While similar, these English terms don't fully capture the action-blocking nature of 于心不忍. You can have a pang of conscience and still do the difficult thing. You can be soft-hearted in general. But when you feel 于心不忍, it is an immediate and overwhelming feeling tied to a specific situation that prevents you from acting. It's less a moral debate and more a visceral, emotional roadblock.
This chengyu is common in both spoken and written Chinese, adding a touch of emotional depth and literary grace to a sentence. It's not overly formal and can be used in many everyday contexts.