Friend takes a bite of the cake: "…yabai."
Friend checks the time: "yabai."
Friend looks at their bank app: "…yabai."
Three completely different disasters and one completely identical word. Welcome to the most over-worked syllables in Japanese.
Yabai started life as underworld slang, somewhere around the Edo period, meaning roughly "this is dangerous, we should leave". Criminals used it about police. It kept that shape for a couple of centuries: bad situation, get out.
Then the 1990s happened, young people got hold of it, and the word did what English did with "sick" and "wicked" — it flipped. Something so good it is almost a problem. A cake so rich it feels illegal. That is yabai now.
The result is a word that carries pure emotional intensity and almost no content. Native speakers decode it entirely from face, tone and timing. Said fast and high, it is delight. Said slowly, dropping at the end, you are about to hear about someone's finances.
For a learner this is a gift, because it lets you react like a human being from day one without conjugating anything. It is also a trap, because if it becomes 30% of your Japanese — and it will try — you will sound exactly like a 15-year-old, which is charming for about a week.
Japanese has a reputation for being indirect and restrained, and then hands you a word that means "I am feeling something enormous and I refuse to specify what". Yabai is not a gap in the language. It is the pressure valve — one socially safe container for every emotion too big to say politely. Which tells you something about all the other words.