If the first syllable of numskull were spelled numb, as it once was, the sense and source would be clearly recognized.
Numb in the skull, with the brain benumbed, dulled.
Actually, however, if we were to follow numb back through the centuries we would find that it should be spelled num after all.
Anciently, our language had a verb nim, meaning “to deprive, to take away.”
A past participle was num, “deprived, taken away.”
But nim has been num from our speech these five centuries.