A trio of t-shirt, and why that would be stupid.

A pair of trousers?

Really?

A pair means two, you understand. But a pair of trousers does not mean two trousers. It means one.
Now, presumably, it's called a pair of trousers because there are two legs to go in it.
But a t-shirt has two holes for arms, and we don't call that a pair of t-shirt.

Admittedly a t-shirt has an extra whole for the head to go through, so perhaps it should be a trio of t-shirt. But then again, if you're a male (or indeed a female who has purchased a SheWee) there is a third hole available on your pair of trousers; you just choose not to use it as often.

The humorless reality of it is that before we had any ability to make clothes properlly, trousers were individual things that you pulled onto your legs. When we manage to figure out a way to make them all in one the linguistic phrase of a pair of trousers just stuck around.

And for some reason has also been applied to any clothing for your bottom half, conjoined or otherwise.

Just another example of English being stupid.