Green’s Dictionary of Slang

(US campus) a joc. reversal of right on! excl.

1972		N.C. Heard To Reach a Dream 206: He took her hand and placed it on the front of his pants. ‘See what I mean?’ he said. ‘Right on, brother!’ ‘Left off, sister!’.
1975		G. Underwood ‘Razorback Sl.’ in AS L:1/2 62: left off interj Word play on right on.