LOL. Watched some Anime, did ya? :) Well, so did I! I think it's close in meaning to "Hontou ni?!" Like: "Oh, really?" Or "Seriously?!"

Hi, Mark's example is definitely fine. You can say "majide" when you ask someone if what he said is truth or not. And it's written in Japanese, "ﾏｼﾞで"。　My English grammer is poor. Sorry. I wish you could understand what I mean. Thanks!!

In addition to what they say, majide is a slang term of "are you kidding?" etc.. and is used mainly by young people.

LOL, didn't they use this phrase in a show 'I Survived A Japanese Game Show' They translated it as 'You got to be crazy' Pretty much you see similar contexts answering your second part of your questions with the other definitions other users have posted up to now :P

In addition, it's typically used by young boys in casual conversation. Girls, depends on how they were raised.