This is a very colloquial expression which is commonly used in conversation especially among youngsters. So it is not appropriate to use in business or at work. You use the expression when you or someone is in trouble. “Sorewa yabai” means “That’s a big problem.” When you are asked by your co-worker if you are busy and you are actually a little busy, you can say to him or her, “Chotto yabai desu” meaning “I am in a bit of trouble.” “Yabai na” is used when you are in trouble and is said to yourself or another.
This can be an exclamatory expression like “Yabai!”, “Yabbe!” or “Yabee!” This is sort of a strong expression, so youngsters use it whenever they discover something unusual. It is similar to “dope shit” in an American slang. When they find cute clothes, very handsome boys or taste a delicious dessert, they say, “Yabai!”