I saw this article on Kuro5hin:
My user-name has been "scotty" since the beginning, well, since I have been empowered to create user accounts on the computers I have access to. I always tried to grab the user-name "scotty" whenever I am signing up to a website, and it has been used for my emails, shell logins, etc. And now days quite a few people just simply called me Scotty, as in the Star Trek "Beam Me Up, Scotty!".
"scotty" has somehow becoming my nick name, but I still prefer people calling me "Scott", instead of trailing with an "tea" sound. I don't have a nick name, and the "y" in "scotty" is simply the initial of my family name, "Yang". Back in CSE when thesis students usually used their first name and concatenated with the initial of their last name to form the user-name. And surprisingly "Scott Yang" has thus used "scotty" as his default user-name.
So now when people asked about my user-name, I'll say "scotty", but pronounced "scott" "why", as the trailing "y" is actually my initial.