Green’s Dictionary of Slang

foolish; ext. in late 20C as doodle-dumb.

1772		G. Stevens ‘Doodle Do’ in Songs Comic and Satyrical 136: All the work they make with wooings, / Couplings, changings, cursings, cooings, / Are but doodling doodle doings.
1979		D. Maitland Breaking Out 169: You are a bloody lop-eared, [...] doodle-dumb, pea-brained, lame-brained, pig-headed, fart-faced flip of a fucking galah!