Originally derived from fourty ounces, bitchin' 40 is something that one says if he/she is very enthused about something, or can be used in place of any adjective with a positive connotation.
1) Hebru: My parents gave me $300 to buy food at my birthday, which means that we can get krunk'd.
Hobo: Bitchin' 40!
2) A woman opens up a christmas present from her husband, they are diamond earings.
Woman: Oh, Frank. I love them, how bitchin' 40 of you.