One of my students asked me the other day why pants (trousers) are plural and I have no idea.
Why do we say “They are pants/trousers” when there is only one and why are they a pair?
“pants” of course is short for a “pair of pants” which is actually an abbreviation for “a pair of pantaloons” As for why they are a pair:
[quote] Before the days of modern tailoring, such garments, whether underwear or outerwear, were indeed made in two parts, one for each leg. The pieces were put on each leg separately and then wrapped and tied or belted at the waist (just like cowboys
[quote=“Mugatu”]One of my students asked me the other day why pants (trousers) are plural and I have no idea.
Why do we say “They are pants/trousers” when there is only one and why are they a pair?[/quote]
Because you have two (a pair of) legs, so you need a pair of pants to cover them.
It’s the same with glasses.
2 legs - pants
2 eyes glasses
1 body - a shirt
1 head - a hat
2 feet - shoes
1 left hand - a glove etc.
It’s logical enough that students can understand it, and easy to remember.
If they persit in arguing, just give them a slap and say ‘because it f-ing is’
Why do Americans call trousers “pant”, instead of “pants”?
It’s the same with “math”.
This thread is a load of pants.
It’s easy to say we say “pants” because we have 2 legs, but how to explain we don’t say “pant” for a “pant leg” when we mean 1 leg ? 
I thought you did.
I say ‘trousers’ and ‘a trouser leg’
Googling seems to suggest
‘trouser leg’ and ‘pant leg’ are more common than ‘trousers leg’ and ‘pants leg’
I’d say it’s a case that those expressions are so uncommon that there is no fixed rule; or that we rarely notice the mistake, a bit like when people write ‘should of…’ or ‘didn’t used to’
In the end I stand by my original explanation ‘because it f-ing is’
edit: Fixed the quote tags
edit 2: I’ve just read BFM’s post again and realised I had misunderstood the point he was asking.
You can say a pantaloon, when talking about the predecessors to modern pants described above, and as pants is simply an abbreviation to pantaloons there is no reason we don’t say ‘a pant’. The reason we have to say ‘a pant leg’ is to avoid confusion with ‘a table leg’ and ‘a chair leg’ etc.
Also
Googled it , and found a plausible explanation on the usage of the word “pant” on Bartleby.Com:
"pants (n.)
is a clipped form of the word pantaloons and is used for various forms of trousers, slacks, or underdrawers; underpants refers primarily to women
Shall we broaden the topic slightly to cover scissors, shears, tongs, pincers, pliers and tweezers?
[quote=“Rik”]Because you have two (a pair of) legs, so you need a pair of pants to cover them.
It’s the same with glasses.
2 legs - pants
2 eyes glasses
1 body - a shirt
1 head - a hat
2 feet - shoes
1 left hand - a glove etc.
:[/quote]
Women have a pair of boobs but they don’t need a pair of bras to cover them. This is one of those “English is not math(s)” things. But you may be right, it might be easier to explain it this way to students instead of going into the etymology of each and every word.
The great thing about pants is that they’re always on sale - 2 for the price of one!
My dog pants.
Do you wear those things? Or do they belong to your dog? 
The dog does, and they belong to me.
(I might wear them at the Happy Hour).
Ok, you wear your dog’s pants, and I’ll wear my pussy hat…
Scissors is more difficult to explain. (shakes fist at Let’s Go writers for declaring them uncountable). For pants I’ve just drawn a pair of legs and “explained” the grammar using jeans and shorts as comparisons.
I heard that originally there was a tribe which only wore a kind of wrapping on their left leg. That was called a “left pant.”
There was another tribe which only wore a kind of wrapping on their right leg. That was called a “right pant.”
Then some of them got married and their children wore “a pair of pants.”
So that is the origin of the expression, and again proves that the English language is quite logical.
I find the word ‘panties’ quite fascinating. http://www.antipanti.com/index.html
[quote=“Bugsy”]Why do Americans call trousers “pant”, instead of “pants”?
It’s the same with “math”.[/quote]
We don’t say “pant”, we say “pants”. (Except when we say “pant leg”).
And we say “math” because it’s “mathematics”, not “mathsematic”! 
(Of course, that doesn’t explain why we say “stats”!) 
You say “math” because you are lazy.
It’s “maths” because it’s “mathematics” and “stats” because its “statistics.”
I have a theory for “Bra.” I don’t speak French, but isn’t “Bra” a short form of “Brasseirie” or something like that which may be plural in French?