Green’s Dictionary of Slang

(US) an expression of agreement, amazement, emphasis.

1947		J.F. Powers ‘the eye’ in Prince of Darkness 183: It’s—un-mis-tak-able ev-i-dence of preg-nan-cy! Get it? [...] Whoa! I don’t have to tell you how that hits the boys at Bullen’s.
1977		Fidrych & Clark No Big Deal 146: [W]hen I won 6-3, I was so happy I walked back to my town. I said, Whoa [ibid.] 228: And here my dad was tellin’ me I’m gettin’ too drunk? When they’re staggerin’? I’m sitting there, Whoa!
1990		W.D. Myers Mouse Rap 101: The same notebook that got Sheri an A only gets me a C. You figure. Whoa!
2008		W.D. Myers Sunrise Over Fallujah 87: We rolled into Baghdad [...] and people on the street were waving to us. Whoa! It’s like winning the Super Bowl or something.
2010		R. Skloot Henrietta Lacks 227: [S]he said ‘Charisma! Woo! I mean, cream of the smooth! Very well versed and knew something about everything’ .
2011		W.D. Myers Cruisers: Checkmate 58: That bathroom is where we found her body.’ It was, like—whoa!’.