Definition from Wiktionary, the free dictionary
- Elder sister of a male
- (by extension, spoken by a male) Unrelated female slightly older than oneself
Usage notes 
- The term 누나 (nuna) is often used by a male to address an elder female as a polite replacement for a second person singular pronoun (너 or 당신).
Alternative forms 
- honorific: 누님 (nunim)