English is a pretty strange language when you think about it.
I mean, we don't move very fast when we're fast asleep, do we?
If we fight with each other, does that mean fight against each other or that we've teamed up together against a common enemy?
Overlooking something can mean that you watch it closely, but it can also mean that you failed to notice something.
Speaking of 'failing to notice something,' here's a question some of us might not have considered before: Why is it called a 'pair' of undies when it's just one item of clothing?
The answer is actually quite simple: because they're underpants and we say 'pair of pants' even though that's just one item of clothing.
So why do we say 'pair' of pants, then?
Okay, maybe the answer isn't actually quite that simple.
See, in the olden days, people didn't have fancy things we have today, like' the internet' or 'Lil Peep' or 'jeans'.
What they did have, however, were breeches (or britches).
Have you ever noticed how men in the majority of historical paintings (such as portraits) over the last 800 years or so look like they're wearing stockings? That would be because they are.
Those stockings were the forerunners to pants in the western world, and they're called 'breeches'.
However, breeches weren't like modern hosiery, because there were two of them: One for each leg. They would be put on individually and fastened around the waist, usually by a drawstring and/or buckle - much like how the sexy 'stockings and suspenders' combo has two single leggings which are, in turn, held up by the suspenders.
In other words, a 'pant' is one of the breech leggings.
So, if you wear two of them, then you're wearing breeches or (wait for it, you won't see this coming) a 'pair’ of pants.
Ok, we’re getting somewhere, but why are they called 'a pair of pants' and not 'a pair of breeches'?
That would be because of medieval pop culture. Yes, there was pop culture before Bieber. There was even pop culture before Britney Spears. The word 'pants' is a shortened form of the word 'pantaloons', a word which refers to a medieval Italian theatre character named 'Pantalone' - a character who wore breeches to show off his wealth.
Over time, pantaloons was shortened to 'pants', because shortening words is a long-lived human tradition - so every time we use the words 'pants' we're literally using slang to refer to a Scrooge McDuck type of character from Italian comedies.
So when you think about it, jokes about underpants are kind of intellectual. Well, sort of.
Side note: Consider pointing that out next time your teacher/parents/whoever complains about how 'the internet is ruining English'. If you do, why not come back here and let us know how it went?
So that's why we say 'a pair of underpants' instead of 'an underpant' - in short, because the pants in question refer to the individual leggings, and the underpants were, and in fact are worn under the pants.
Most modern underwear has evolved over time from the type of underwear known as 'long johns', which are literally full-length underpants. They're still around, too - but now we call them 'thermal underwear'.
Hey, like we said: English is a very strange language