Origins date back to at least the very early 60's. Great, the best, hot, excellent, awesome, unbelievably good. In the 60's and 70's was often combined with the word "ass", as in "bitchin-ass", to describe the ultimate, supreme, primo, or a great looking sexy girl.
1. Check out that bitchin car!
2. We had such a bitchin time at the concert.
3. Wow what a bitchin-ass babe!