マジ
Maji
まじ
Comes from 真面目 (majime = serious/earnest) and means "really," "seriously," or "for real." "マジで？" (maji de?) is one of the most common reactions in Japanese conversation — equivalent to "seriously?" or "for real?" As an adverb, マジで can attach to basically anything: マジで美味しい (seriously delicious), マジで無理 (seriously cannot handle this). While it started as youth slang, it is now understood across all generations. You can also use just マジ on its own — "マジ？" (really?) or "マジだよ" (I am serious).
Examples
マジで？来月から給料上がるの？
Seriously?! You are getting a raise next month?
Hearing good news from a friend
あの映画マジで面白かったから観てみて
That movie was seriously so good — you have to watch it.
Recommending a movie to a friend
マジで眠い。昨日3時間しか寝てない
I am seriously so sleepy. Only slept three hours last night.
（勝手に冷蔵庫にとっておいたデザートを家族に食べられてしまったことに気づいて）え、マジか。信じられない。
Wait, seriously? I cannot believe it. (Upon discovering a family member ate the dessert you had been saving in the fridge)