An all-purpose Japanese intensifier for anything extremely good or extremely bad — context and tone decide which.
やばい (yabai) is Japanese for when something hits so hard you can't find another word. It's the linguistic equivalent of your eyes going wide — works for 'this is incredible' and 'this is terrible' depending entirely on your tone and face. Young people use it as their default intensifier for anything extreme.
The word crawled out of Japan's criminal underworld in the 1700s as a warning that police were near — pure danger, pure risk. For centuries it stayed that way. Then in the 1980s, Japanese youth flipped it. If something dangerously bad is yabai, why can't something dangerously good be yabai too? The positive meaning spread like wildfire while the negative one refused to die. Now both live in the same word, sorted by context.
This creates a generational split you can hear in real time. A teenager shouts 'yabai!' at a perfect sunset; their grandmother hears 'dangerous' and worries. Formal settings reject it entirely — it's pure casual speech, marked as youth slang. But among young people, it's so grammaticalized it has clipped forms (yaba, yabee) that work like verbal punctuation. The criminals who invented it would never recognize what it became.