Colloquial/abbreviated expression meaning 'serious/not joking'; used to emphasize or as an exclamation (e.g. 'Really?').
まじで行くの？今日の夜？
Are you seriously going? Tonight?
試験に受かったよ。— まじ？おめでとう！
I passed the exam. — Really? Congratulations!
彼の話はまじだから、冗談だと思わないで。
He's serious, so don't think he's joking.
マジで疲れた。今日は早く寝る。
I'm really tired. I'm going to bed early today.