originally, pants were two separate legs with no middle bit. Then they got a middle bit and turned into one garment, but kept the plural name. Interestingly in the fashion and tailoring world, there does seem to be a move towards talking about "the trouser" or "the pant"
So the under garments had no middle bit... and "trousers" are plural, implying they were once separate items too... so just what exactly covered the middle bit?!
if you leave it as singular it is what you do when the plural is removed, when accompanied by another and in a romantic setting...Leave the word alone, what harm is it doing.. by the way bupf where did you find my question on Who Decided??