Headcanon: The first time Mycroft swore in front of Greg he didn’t realise and carried on ranting. That is, until the next morning when Greg’s alarm sounded and it was Mycroft’s shouted slurs. Greg was still late for work because he couldn’t move for laughing.
Headcanon: In a very concussed state Mycroft once explained to Greg that he had “nearly been run overed” before he became completely mortified with himself and added “I need to grammar better”. Greg may or may not have recorded the whole ordeal.