'smooth-headed' originates from the combination of 'smooth' and 'head,' where 'smooth' meant 'having an even and regular surface' and 'head' referred to the upper part of the body.

'Smooth' and 'head' have been used in English since Old English, and their combination into 'smooth-headed' is a modern English formation.

Initially, 'smooth' meant 'even surface,' and 'head' referred to the top part of the body. The combination retains this meaning in modern usage.