The topic might have come up here before of words and expressions we know only from reading and not from conversation, so we just have to guess their meaning from context.
"Tow-headed" is definitely one of those for me. I'm not sure if I've ever heard anyone say it, but I've certainly come across it in novels and such.
It was often in reference to children ("a tow-headed young lad") so I assumed, incorrectly as it turns out, that it meant "having messy hair".