I'm sure I'm not the first to comment on this, but the plurality of trousers, and even more, of underpants, is one of the English languages real oddities.
I was reminded of this, strangely enough, because as I worked back to work this morning, I came across a crumpled pair of underpants lying on the ground. But there was only one object there at my feet.
I can appreciate that trousers might have a sense of plurality in that there are two legs involved, even though they're part of a single unit overall (well, they're not overalls, but you get what I mean). But underpants? Where is there anything double about them? Two holes for the legs, maybe, but we don't call them underholes, for all that. And since many underpants (or, if you want, shorts, in the US) are little more than a wrap-around to hold the bum and the other bits together, how is there any sense of them being more than one unit? Shorts, as a word, is just as bad. Surely there's only one short, as there is only one underpant?
So, in fact, as I was walking back to work, I found not a pair of underpants - what's paired? - but a single underpant lying unhappily where it was dropped. And where it was dropped is an altogether different question.