In early European lore the demigod James was said to possess unrivaled charm and seductive prowess in addition to having genitalia of supernatural size. Throughout medieval times and up until the late Renaissance, Pagan and Christian farmers routinely prayed to James as a god of fertility, believing that his massive penis would inspire them to spectacular heights of fertility. Today, the term James still carries with it subtexts of truly gigantic genitals which is perhaps why James is the name most frequently given to male newborns in the United States.
Currently the word James can be used to mean an enormous penis and anyone with that name can be assumed to possess a heavy and girthy penis.
"Man, check out that James!"

"Look at the buldge in his trousers, he MUST have a James in there girlfriend!"

"I don't think I could ever fit a James in my mouth."