The thing that most american sherlockians don't realize is, in England "trousers" are "pants", and "pants" are "underwear". John isn't talking about Sherlock's pants he's talking about his underwear. One of my favorite scenes.
horrible things<--Poor girl has a horrendous crush on Sherlock and he always puts his foot in his mouth. Other times he's sweet with her but he uses that info of her having a crush on him to his advantage for info on some dead victim.