Seats in a theater, usually for a live show of some sort, that are so high and far away from the stage that you get a "nosebleed" from the high altitude.
"Where were you for poker night, man?"
"Oh, man - My girlfriend dragged me to the opera! The show was horrible AND we had nosebleed seats!"
"Rough, dude."
Ok ok so its not a word no biggie. If you listen to any hip hop at all you have probably heard this line upwards of 20 times. It comes from the Mobb Deep song "Shook Ones". this line is probably more popular than any other line in hip hop.
"Rock you in ya face stab your brain witch yer nosebone
You all alone in these streets cousin Every man for dey self in dis lan' we be gunin'
an keep dem shook crew runin...."
-Mobb Deep "Shook Ones"
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.