You might hear this sound quite a bit over the next few days. Perhaps someone might be trying to give you a scare. Or perhaps they might be expressing disapproval at your choice of Hallowe’en costume. But why do we always use boo to frighten or to show disapproval?

As you might imagine, it’s pretty straightforward. It begin as boh in the 15th century, meant to be a loud and startling sound. This all seems to be down to the specifics of the sound. The initial B sound (known as the voiced bilabial stop) givs it an explosive start and the resulting long vowel sound (originally Oh and now Oo) provides a deep, loud finish.

This combination seems quite universal as a method of startling someone, as the Oxford English Dictionary also notes the similar boare in Latin, and boaein in Greek.

But why do we use it to show our disapproval? This is quite different from scaring someone. It originated in the 19th century in British theatre-going circles, and quickly spread from there. It was meant to be imitative of cows, probably because the sound they make (moo, in case you were wondering), already kind of sounds like a grunt of disapproval.

What’s interesting about boo is that there’s no complex chain of etymology: it just produces a direct emotional response, and that’s what makes it effective. But I think that’s something that happens with all words. Sure, we can see how words came from older words in older forms of English, or from different languages. But I also think words only come into use in the first place if the basic sounds they make produce some kind of effect on us. Some sound soft, or hard, or pleasant, or cold, and that influences how we use them and the meanings we give them.

But of course few words have the immediate impact boo has. Or boo-urns. I was saying boo-urns…