Green’s Dictionary of Slang

1. (US) something, or somebody, impressive, sensational, successful; also as adj.

1917		F. Packard Adventures of Jimmie Dale (1918) I vi: The Wowzer, beyond a shadow of doubt, in his own profession stood upon a plane entirely by himself.
1924		L. O’Neil ‘These Degenerate Days’ in Dinkum Aussie and Other Poems 136: It’s fallen on a wowser world! It’s plain as copperplate! / For using sinful language now they give the cook the gate!
1951		P. Whelton In Comes Death 148: [She] Certainly was a wowser on that Gay Nineties stuff.
1959		S. Bellow Henderson The Rain King 167: So, it’s a real wowzer, Your Highness.
1978		S. King Stand (1990) 1308: Some of the rifles are wowsers. Scope-equipped.
1983		S. King Christine 27: The first full-scale argument in the Cunningham family that I had ever seen [...] And it surely was a wowser, at least ten on the Richter scale.

2. (US) used as excl.

2005

D. Jenkins Slim & None 115: I said, ‘What happened to Ashley, if I may ask?’ ‘Oh, wowser. Another bump in the road. Like the man said, ‘You can’t live with ‘em’ .