I'm guessing it starts with a "b"? If so, I understand why it could bother you, as I assume you're female, so it might seem like a sexist thing? At the same time, if my T (who's male, but not that much older) said something like that to me, I'd likely be fine with it because of the relationship we have. Like I would figure he was trying to be funny and/or encourage me to let my anger out in the letter (since I'm not good at expressing anger). If it starts with a "p" (which I don't think makes sense in this context, with the angry letter), then my answer would be different. If it's something else entirely, then I don't know!