Green’s Dictionary of Slang

(orig. US) person-to-person, intimate or confrontational, e.g. a fight.

1977		New Yorker 17 Jan. 73: Politics is a one-on-one relationship, because if you don’t feel some warmth and friendship with people, then you ought not to be in politics.
2006		G. Malkani Londonstani (2007) 101: Dey tryin 2 keep dis shit one-on-one, u get me.