悉心 is an adverb meaning 'with great care and attention' or 'meticulously.' It combines the character 悉 (xī), meaning 'thorough,' 'complete,' or 'to know fully,' and 心 (xīn), meaning 'heart' or 'mind.' Together, they convey the idea of engaging one’s full mental focus and heartfelt effort — not just physical action, but deep, attentive involvement. It emphasizes sincerity, diligence, and emotional investment in a task.

This word is commonly used in formal, written, or respectful contexts — especially when describing care for people (e.g., patients, children, elders) or meticulous work like research, teaching, or craftsmanship. It often appears before verbs such as 照顾 (to take care of), 培养 (to nurture), or 研究 (to study). While it can modify verbs directly, it rarely stands alone and almost never functions as a noun or adjective.