Green’s Dictionary of Slang

berk adj.

stupid.

1989		M. Amis London Fields 43: He will have a rule about it – Keith, with his tenacities, his berk protocols, his criminal codes.
1999		Observer Rev. 4 July 11: One of Melvyn Bragg’s berk psychologists will probably argue that procrastination is a valuable facet of the human mind.