Green’s Dictionary of Slang

(US teen) a general excl., either of approval or of vaguely non-committal agreement.

1933		Phila. Daily Bulletin 1 Feb. 6: Yowza--Yes, sir.
1937		A.E. Duckett ‘Truckin ’round Brooklyn’ in N.Y. Age 10 Apr. 7/1: Yowsah, the Chrysalis kids are all right.
1999		Indep. Rev. 6 Nov. 8: ‘Love-bomb your sports department,’ she suggested. (Yowzah!).