BDD is short for Big Dick Daddy. Often used as an acronym to describe someone who you want to throw you across the room and shove their monster cock in you.
Girl 1: Anna is so lucky. Her boyfriend Taj is a BDD.
Girl 2: What is a BDD?
Girl 1: Taj is what you call a “Big Dick Daddy.”
A trans person who passes but doesn't think they do on account of body dysmorphia, perfectionism, fishing for compliments, or most likely some combination of all three.
Alice never gets misgendered but she insists that she is a massive hon. Textbook BDD passoid.