When you are told by someone that something amazing just happened, but they won't tell you. You know how when guys are just about to get it in, they get all excited?
well, if they don't get it in, and they're still excited, they get blue balls. or it just hurts them a lot, because they can't release.
Mentally blue balled means that you got all excited and then didn't complete with the exciting thing/action.
Jane: OMGAH,Jenny! You'll NEVER guess who's transferring into our school next week. You are gonna FREAK the fuck out! OMG i could barely believe it myself.
Jenny: Tell me! Tell me!
Jane: Nah, chill.
(Jenny is mentally blue balled)