全身心 (quán shēn xīn) means 'wholeheartedly' but specifically implies total devotion of both body and mind — you give your full physical and mental energy to something. It is broader than 全力 (quán lì, 'with all one's strength'), which focuses on physical effort, and 一心 (yī xīn, 'single-mindedly'), which emphasizes mental focus. 全心全意 (quán xīn quán yì) is a close synonym but often carries a more emotional or heartfelt tone.