Green’s Dictionary of Slang

astounding, amazing, overwhelming.

1933		T.T. Flynn ‘The Deadly Orchid’ in Goodstone Pulps (1970) 104/2: Forget-me-not eyes, a knock ’em dead face, and a clinging manner.
2004		C. Brookmyre Be My Enemy 99: The former Scotland Tonight frontwoman was not exactly a knock-’em-dead beauty.