I still think that 'plonker' as term for stupid person retains an overtone of sex-organ [again, compare cunt & twat & prick]. There are, after all, few English words of one sole definition, and many whose derivations may be disputed, or from more than one source.

& anyone who thinks different is a pillock!*

≈M≈




*which Partridge gives as undisputedly an obsolete word for the male organ...