Green’s Dictionary of Slang

cuckoo adj.

also coo-coo, cookoo, cuckoo’s nest, koo koo
[cuckoo n.1 (2)]

[mid-19C+] crazy, eccentric, insane.

In phrases

knock cuckoo (v.) (also knock coo-coo, slap cuckoo) [1910s–30s] (US)

1. to knock out, lit. and fig.

2. to amaze, to astonish.