Literally 'sour heart' — that specific ache when you feel sad, touched, or moved to tears by someone's hardship. Not quite sadness, not quite pity — a uniquely bittersweet feeling.
心酸 describes empathetic sadness — you feel it when hearing about someone's struggles or remembering hard times. It's different from 伤心 (sad) or 难过 (upset). 心酸 has a quality of 'my heart aches for this,' often accompanied by the urge to cry.
