Green’s Dictionary of Slang

1. of a fight or competition, one-on-one, thus open, honest.

1974		D. Jenkins Dead Solid Perfect 136: ‘[I]t’s just gonna be him . . . and . . . me. Heads up. [...] And I've been to all the big rodeos in that connection’.
1994		N. McCall Makes Me Wanna Holler (1995) 129: I probably couldn’t beat him in a heads-up rumble. I’d have to bum-rush him.
2007		N. McCall Them (2008) 27: We took it outside and everybody [...] let us go, heads-up.

2. alert and skilful, esp. in sport.

1970		J. Bouton Ball Four 172: We’re playing heads-up ball [...] and when the other team makes a mental error we take advantage of it right away.
1977		Fidrych & Clark No Big Deal 112: [H]e wasn’t heads up. Y’know, he just got caught dazin’.