Sometimes used in anime and manga, nosebleed means to become sexually excited. It's usually embarrassing and something the character will probably try to cover up. Though it normally is shown happening to male characters, female characters have also been shown having nosebleeds.
It is also a term used by fans, regardless of their type of fandom, to express finding something exciting sexually or hot.
Sailor Venus had a nosebleed just thinking about Yaten Kou in the anime version of Sailor Moon.
He quickly covered his face in an attempt to stop his nosebleed.
Girl 1: Oh my gosh, did you see the latest manga chapter?
Girl 2: Yeah, Ed looked so sexy in the chapter cover, I totally wanted to nosebleed.