やばい！

Yabai!

Oh no! / Wow, awesome!

Word for word

“Dangerous.” (originally)

Politeness: Slang

Very casual and trendy. Friends only, and it dates quickly.

When you would say it

Friends only. Both danger and delight.

Worth knowing

Meaning depends on tone: ヤバい、おいしい！ is good; やばい、遅れる！ is bad.