The t-dot is short for Toronto. Its slang used mainly by wiggers like Sean Desman and "gangsters". I am a wigger, so I use it, but whatever. I'm an exception because I be cool, yo.
"Heyuh girlie! A/S/L?"
"Yo yo am 16/f/T-dot"
"YOU REPPIN T-DOT"
"Ye ye." *smiles proudly*