Green’s Dictionary of Slang

(US) fearless.

1958		R. Frede Entry E (1961) 55: Stew Trent, the sometimes wrestler, but all times drinker and whorechaser and general guts-high gung-ho aborigine.
1967		W. Murray Sweet Ride 206: Denny Maguire? He was guts up, man.
1972		D. Jenkins Semi-Tough 275: ‘et’s get it, let’s get it. Guts up time now. This is a gut check. Gotta have it’.