There are many theories about the origin of やばい. The strongest theory is that is comes from the word やば, a な-adjective that mean “not good” or “inconvenient”, and over time turned into the い-adj, やばい. We no longer use the な-adjective form, やば - やばい has basically taken over its meaning and place in the Japanese language. やばい is also a very flexible word, and can be used in a variety of ways depending on the context.
Some dictionaries will note that やばい means 危ない, or “dangerous.” This is true, except the connotation is much closer to “risky” rather than “dangerous”. For example, we cannot scream やばい at a kid that jumps out in front of a car. The basic meaning of やばい is “not good” inherited from the original word やば, therefore, this やばい means “risk of sth goes wrong (=not good)”.
やばい can also mean “full of problems” or “terrible”, as in the sentence, 田中さんの仕事はやばい - “Mr. Tanaka’s work has a lot of problems”.
In 1980’s, young people also started using やばい to mean “a bad appearance.” If someone told you your hair was やばい, then you should check yourself in a mirror because it means that your hair looks bad today.
Generally, やばい takes one a negative connotation. However, in the 1990s, it started to take on a meaning of “extremely” or “very”. We can say このラーメンはやばいくらい美味しい or “This ramen is extremely tasty.” Using this meaning, やばい can take on positive or negative meanings.
We do not use kanji for やばい, however you may sometimes see it written in katakana, as ヤバい.
やばい is mostly used by young people - you could say it is basically a slang word. So make sure you don’t use it in a formal environment or to people in a higher social status. Otherwise you may get a few stares!
I slept over! I may be late for work!
Is this milk old? The smell is bad.