The word 'towhead' is a compound formed within English from 'tow' and 'head'. 'Tow' (Old English tōw, meaning 'fibre for spinning') originates from a Proto-Germanic root related to making or preparing. Its application to hair color is metaphoric, comparing the pale, flaxen color of spun flax fibers to a particular shade of blond hair. The term 'head' (Old English hēafod) comes from Proto-Germanic haubudą. By the early 19th century, 'towhead' emerged in American English as a descriptive term, first for the hair itself and then for a person possessing it, solidifying its place as a fixed idiom for very light blond hair.

This is a compound noun formed from two English words, but its modern meaning is idiomatic and not directly compositional from the senses of its parts. 'Tow' refers to the coarse, short fibers of flax or hemp prepared for spinning. 'Head' refers to the part of the body containing the brain. The compound functions as a single lexical unit.