This term originated on www.supraforums.com as a strange slang for the Toyota Supra turbo. Typically, a "Supdawg" is of the 1994 or newer model.
"Damn, you just got smoked by the supdawg!"
"Tonight we're taking Johnny's mustang out and we're gonna hunt us some supdawgs!"
Supdawg is an American slang greeting. It is a heavily corrupted contraction of the properly phrased question "What is up, Dog?". It's genesis is rooted in the American inner city neighborhoods of the 1980s where it has fluorished in the Hip Hop music subculture. It is notably in extremely heavy usage in the West Loop area of downtown Chicago centering on the intersection of Clinton and Van Buren streets.
Person 1: Supdawg!
Person 2: Supdawg!
Person 1: Supdawg?
Person 2: Nothin'