The origin of Maji is Majime (真面目) which means seriously (真剣). so your translation "really" is correct. Yabai is a slang of 危険 or danger, so it originally meant things-not-so-good, however recently, it's becoming to mean good things sometimes. This is seen with young generation mostly.
Say, when you go into a restaurant, you take some dish, the taste is so good that you are surprised, then you can say これやばい.
These are kind of teenagers words, so it would be better not to use them when you speak with someone not closed or older people.