Green’s Dictionary of Slang

an excl. of encouragement, approval: excellent! perfect! exactly right! orig. black use but taken up by white hippies, radicals etc.

1911		H.W. Odum ‘Folk-song and Folk-poetry as Found in the Secular Songs of the Southern Negroes’ in Jrnl American Folk-Lore July-Sept. 292: Railroad Bill was [a] mighty sport, Shot all [the] buttons off [the] high sheriff’s coat, Den hollered, ‘Right on, desperado Bill!’ .
1968		‘Sl. of Watts’ in Current Sl. III:2.
1970		B. Seale Seize the Time 122: A lot of the brothers said, ‘Right on time. Huey. Tell it. Right. Run it down, Huey.’.
1971		J. Mandelkau Buttons 90: The whole house was full of naked people getting it on. Right on, brother!
1976		R.M. Kavanagh Survival in Kavanagh S. Afr. People’s Plays (1981) 143: Right on, brother Vusi.
1989		T. Blacker Fixx 102: Never trust anyone over thirty? Right on, man. I could dig it.
1998		(con. 1960s) G. Washington Blood Brothers 8: Right on, baby, right on.
2005		L. Theroux Call of the Weird (2006) 164: Right on, man! Keep this town real!
2012		M. Lacher On the Bro’d 204: [H]e started hinting [...] that he was a gay dude [...] Derek was all like, ‘Right on’.