Someone calling Sherlock pretty boy in a derogatory way and John just smiling menacingly as he walks up to them and grabs their shirt collar, bunching it tightly in his fists to pull them in close and whisper:
"He is pretty, but I’m the only one on the planet who can call him that. If I hear it from your mouth again, I guarantee you- his face will be the last beautiful thing you get to see."
And the guy calling John mental and scurrying away while Sherlock looks confused and asks John what he said, but John just pats his back in response. (◡‿◡✿)