What does the name "Uncle Six" mean, if anything?

Wu Assassins features a Triad leader named Uncle Six. Everyone (from his adoptive son to police officers) refer to him as "Uncle Six", but that does not appear to be his real name, especially since Zan also calls him a "Big Brother", making "Uncle" sound like a title. I am thus wondering what, if anything, is the meaning of this nickname.

The "Uncle" part implies a form of authority, which fits his character, but I’m not sure about the "Six" – sixth Triad leader? Allusion/wordplay from Mandarin/Cantonese/other?

Browsing around, I did find that Run Run Shaw, a 20th century Hong Kong businessman, was also nicknamed Uncle Six, but I see zero connection between this man and Wu Assassins.