Green’s Dictionary of Slang

an affirmative excl., absolutely! sure!

1970		D. Mitchell Thumb Tripping (1971) 5: Gary [...] pronounced it ‘Wo-o-ow, righteous!’.
1993		(con. 1975) K. Scott Monster (1994) 10: ‘You got eight shots, you don’t come back to the car unless they are all gone.’ ‘Righteous,’ I said.
1997–2000		College Sl. Research Project (Cal. State Poly. Uni., Pomona) 🌐 Righteous, Righteous (interjection) Right; yeah.