Green’s Dictionary of Slang

knock in the cradle n.

a fool.

[UK]B.E. Dict. Canting Crew n.p.: Knock in the Cradle, a Fool.
[UK]New Canting Dict. [as cit. c.1698].

In phrases

have a knock in the cradle (v.)

to be stupid.

[UK]Wandring Whore I 12: My Maiden-head? He that think’s I had my Maiden head after thirteen, has had a knock in the Cradle, and merits cutting of the Simples.
[UK] ‘Knave out of Doors’ in Rump Poems and Songs (1662) ii 27: She and her Maid gave him the Whip; / And beat his head so addle; / You’d think he had a knock in the Cradle.
[UK]Dryden Sir Martin Mar-all IV i: You are one that had a Knock in your Cradle, a conceited Lack-wit.