TOWHEAD or "flaxen haired'

A towhead is a person with very light, blond or yellow-colored hair. The adjective form is towheaded, and either word may be hyphenated as in tow-head and tow-headed. Towhead is first seen in 1830. Tow is another word for flax, jute or hemp that is ready for spinning. Flax and hemp that is prepared for spinning is light-colored, hence a person with hair that is exceedingly light-colored or yellow would be a towhead. The word tow meaning fiber ready to spin appears in the fourteenth century, probably from the Old Norse word to, meaning uncleaned wool or flax, unworked fiber of thread. Towheaded children’s hair often darkens as they age into adulthood.
http://grammarist.com/words/towhead/

In colonial times, families grew their own flax to make into fabric for clothing. Transforming the flax into thread was a complicated, involved process with many time-consuming steps. After the flax was harvested, it was soaked in water for several days to soften it so the inner fibers could be removed from the stalk. To separate the long, thin fibers from the shorter, coarser ones, the flax was pulled through a bed of nails or combed in a process called “towing.” The shorter fibers that were extricated were of a lesser quality and were called “tow.” This led to the term “towheads” to describe people, particularly children, whose hair resembled these strands.

tow·head
noun  tow·head \ˈtō-ˌhed\
Tow headed is literally “flaxen haired”. This meaning of tow comes from Middle Low German touw (which means “flax, hemp fiber”). This probably went back to the prehistoric Germanic base *tow-, *taw “make, prepare” (source also of English tool), in the sense “make yarn from wool; spin”.

From The Grammarphobia Blog
Why is a blond kid a towhead?