费心 is a verb meaning 'to take trouble' or 'to worry (about something)'. It combines 费 (fèi), meaning 'to expend' or 'to cost', and 心 (xīn), meaning 'heart' or 'mind'. Literally, it suggests 'expending mental energy' — implying effort, concern, or careful attention. It often carries a gentle, appreciative tone when thanking someone for their effort ('Thank you for taking the trouble'), or a slightly apologetic tone when asking for help ('I hate to trouble you...').

This phrase is commonly used in polite, everyday speech — especially in requests, thanks, or expressions of concern. It’s rarely used in formal writing alone but appears frequently in spoken Chinese and informal correspondence. Note that 费心 is not a noun; it functions as a verb phrase and typically appears with auxiliary verbs like 请 (qǐng), 不要 (bù yào), or 感谢 (gǎn xiè), or as part of a predicate (e.g., 别费心了 — 'Don’t bother').