This was a long running thing ... my wife used to use "pants" in a lot of references to me, such as Mister Smarty Pants, Mister Crazy Pants, Mister Silly Pants, etc. Eventually it was reduced down to simple "Mister Pants" and ultimately just "Pants". Then it became generalized phrase rather than an alias, as in "that person is being such a pants."

Pants are also found in the Bible: "As the deer pants for streams of water, so my soul pants for you, my God." [Psalm 42:1] So we also have deer wearing pants, and we know that pants are clothing for the soul. Fuck chicken soup.