Y/N

Used mostly in fanfictions to describe a characters that is you.

Means Your Name.
(... no not the wonderful anime movie 'Your Name' that even made the most macho ones of us shed a tear)

A:He sat next to Y/N and Held her hand.
.... what the f- does that mean... Yoona?
B:What? No. It means 'Your Name' ,

you dummy.