"Origins of words" by Susie Dent, Countdown 11/12/17 (letting hair down, get in someone's hair, split hairs, hairy, hair of the dog)

I'm going to talk about hair and the numerous expressions in English involving hair. And I'm going to start with letting your hair down, which seems quite obvious, really, on the face of it. But actually, in the 19th century, it was letting down the back hair. And it involved not the hair on your back, but these very elaborate tresses of hair that, of course, women would wear in these days. And only in the informal, relaxed atmosphere of the home were they able tolet these tresses down. And when you get in someone's hair, you're being an annoyance. Not quite sure about this one, but we think it's due to the irritation of head lice, that was the original meaning. So not a particularly pleasant one there. People who split hairs quibble over insignificant details. And that image, between painstackingly dividing a single hair, which of course is almost impossible, and making small and slightly over-refined distinctions, has been around for centuries. In fact, Shakespeare used it. In King Henry IV he writes, "I'll cavil on a ninth part of a hair', meaning, "I'll argue over everything", a tiny portion. And then we have hairy. If we describe something as hairy, it's quite scary - you might have a hairy flight, for example. That sense of hairy is probably simply a version of hair-rising, makes your hair stand on end. Which is also, of course, behind horrible and horror. It goes back to a Latin word meaning stand on end, with the idea that your hair bristle at the sight of something truly terrifying. But possibly my favourite origin, one of my favourite origins in English, totally, actially, the hair of the dog. We talk about that when we have a tipple the morning after the night before, in order trying to try and cure a hangover.  It goes back to the full expression, the hair of the dog that  bit you, and that goes back quite a long way to people who were perhaps bitten by rabid dog, and the belief that if you could chase that dog down, pull out a bit of its hair, make a poultice out of the hair and put it over the wound, it would cure your rabies. And? of course, over time, that was metaphorically then applied to alcohol. But it begut with something very, very literal, and very real hairs of an actual dog.