千金 has three distinct but related meanings. Literally 'a thousand pieces of gold', it came to mean 'a huge amount of money' or 'very expensive'. In polite speech — especially in formal, classical, or courteous contexts — it's an honorific for someone else's daughter (like saying 'your precious daughter'). It can also mean 'invaluable' or 'extremely precious' as an adjective-like noun. Use the daughter sense only when addressing or referring to another person's daughter with respect, not your own.