Green’s Dictionary of Slang

wowser n.2

also wowzer
[SE wow!]

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

[US]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.
[Aus]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!
[US]P. Whelton In Comes Death 148: [She] Certainly was a wowser on that Gay Nineties stuff.
[US]S. Bellow Henderson The Rain King 167: So, it’s a real wowzer, Your Highness.
[US]W.R. Burnett Widow Barony 12: No young doll. But with a little animation and some decent clothes she’d be a double-barrelled wowser.
[US]S. King Stand (1990) 1308: Some of the rifles are wowsers. Scope-equipped.
[US]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.

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’ .