It short for my full name - Thomas Thomassen. (Oh yes it's true - and that name repeats itself down my history tree - names, as it turns out, are inheritable. Though they usually skips a couple of generations.)
Got the nick when I went to England - we where three Thomas' in the house. One was called Thom, the other by his last name, and I became ThomThom. No one used 'Thomas' in England - they like to shorten words there. Even ThomThom got shortened, but since Thom was already used, it became TT - then even that was shortened to just T. "What some more tea, TT? hihihi..." ugh....
So there!
What's your story? π