Queen's knuckle
Historically, a condition where one's finger joints are severely enlarged, particularly the proximal interphalangeal joint (middle knuckle). The term is thought to originate from Queen Victoria's preference for the condition when choosing her bath valets. Today, it generally refers to the middle knuckle of the middle finger.
My bath valet filled the bath to a depth just shy of a queen's knuckle, and then my bath valet furiously pleasured me.