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.
"so did you know this scene was andrew's audition for moriarty"<- I can honestly say this is probably exactly how that went << do you want more adorable Andrew Scott? He plays this Welsh guy who is just plain adorable!