Yabai’s original meaning is “inconvenient” and “dangerous”. It of course still has original meaning and has new meaning at the same time as slang.
Some people living in Japan may have chance to listen to Japanese people saying Yabai. This is mainly used in among close friends and very informal situations. Especially, young people say Yabai when they are impressed and surprised at something. This is used independently.
For example, when you eat delicious cake, you can say “Kore Yabai!” which means “this is delicious!” When you see a really handsome guy on the street, you can say “Kare, Majide Yabai!” which means “He is so cool.” (See the meaning of Majide)
People also say “Yabê” instead of “Yabai”. “Yabê” is a kind of a derivative of Yabai. Both men and women can use this, but not suitable for women to use because it sounds a bit coarse in my opinion.