まじ/まじで・maji/majide = seriously, really, truly
You can use this to say:
- I was so scared, seriously!
- Do you really mean that?
- Stop doing it, I'm serious.
EXAMPLES
Really

あのおばけやしき、まじでこわかったよ。
(Ano obakeyasiki, majide kowakatta yo.
(Ano obakeyasiki, majide kowakatta yo.
ああ、そうなんだ。じゃあ、わたしはいかない。
(aa sōnanda. jā watasi wa ikanai.)
(aa sōnanda. jā watasi wa ikanai.)

Seriously

げーむ、やめなさい!まじで!
(Gēmu, yamenasai! Maji de!)
(Gēmu, yamenasai! Maji de!)
あと いっかい だけ やらせて!
(Ato ikkai dake yarasete!)
(Ato ikkai dake yarasete!)

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!!
Let's learn Japanese!