Обалдеть is, literally, to become a blockhead, an idiot. It's a colloquialism.
1) It can be used as an exclamation:
- "Обалдеть!" - "That's crazy!", often meaning "crazy awesome", or just something unexpected.
It is actually a shortened form of:
- "Обалдеть можно!" - literally "You can becume dumb (from this)", "This can make you dumb", meaning that teh thing we are talking about is something unexpected.
2) It can be used in phrases in a similar meaning:
- Стас просто обалдеет, когда узнает!
The speaker expects Stas to be very pleasantly surprised with the news.
3) When we say that someone "обалдел" in past tense, we often mean that someone has become extremely brazen, to the point of losing common sense.
- Представляешь, он без спроса взял деньги, отложенные на отпуск, и купил себе электрогитару!
- Совсем обалдел!