Green’s Dictionary of Slang

(US) wonderful! great! esp. in surfer use, subseq. adopted by teen girls of 1980s California.

1961		F. Kohner Gidget Goes Hawaiian 22: ‘And I’m going to Hawaii,’ said Wally. ‘Bitchen!’.
1965		Poston & Stillman ‘Notes on Campus Vocabulary’ in AS XL:3 193: It can serve both as an interjection (bitchin’, man! = ‘Great!’) and as an adjective.
1965		P. Wylie Esquire July 44–45: bitchin—good, great!
1967		W. Murray Sweet Ride 88: Fab! Gear! Bitchin’!
1984		S. King Thinner (1986) 271: ‘Bitchin’!’ she said, laughing.
1998		Source Oct. 120: Before he can say ‘Bitchin’ dude’.
1999		Indep. Rev. 10 June 4: More new words clocked in the 1999 Concise Oxford Dictionary. Bitching: excellent (youth).
2001		J. Stahl Plainclothes Naked (2002) 139: ‘Book some range timer. Get in some target practice.’ ‘Bitchin’!’.