What is Bag Of Pants?
1.
Useless. As would be a bag of pants, not fulfilling their function, but just hanging around in a bag.
"Sorry we haven't got any."
"Well that's just a bag of pants, isn't it."
2.
Someone disposed to wearing his low-riders at knee level thus completely revealing both underwear and outerwear.
I wonder if that bag of pants is wearing a thong?