Shortened form of bigus dickus, a character from the Monty Python film, Life of Brian. The humour of the scene is based on the character so named being unaware that the fake Latin name suggests the character has a large penis
I have a vewy gweat fwiend in Wome called Bigus Dickus (dialogue from Life of Brian)
Guy1: Wow, Steve had intercourse with a girl and another guy at the same time!
Guy2: It's because he's a biguy.
Guy1: Oh, I see... But how does one become a biguy?
Guy2: Follow me to my room, I'll show you.