"Maji/majide" is an abbreviated form of the word "Majime-serious". As you may already know, Japanese people abbreviate words/sentences so often and many of the shortened words have already become "official". "Maji" is one of them.
When "Maji" is used in a sentence, (most of the time) it's more natural than using "majime"
ああ、そうなんだ。じゃあ、わたしはいかない。
(aa sōnanda. jā watasi wa ikanai.)
Keiko
(Oh yea? Then I won't go.)
Seriously
Grace
げーむ、やめなさい!まじで!
(Gēmu, yamenasai! Maji de!)
(Stop playing the game! Seriously!)
あと いっかい だけ やらせて!
(Ato ikkai dake yarasete!)
Keiko
(Let me do it just one more time!)
A tip
When you ask someone "are you serious?", you can say "Maji?" or "Maji desuka?" It's a generally accepted term. Many people think of "maji" as a relatively recent term, but its origin goes back to Edo era (1603-1868). Majide!!