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2.
A casual greeting used when one is chatting or talking. It means "Sup Dude"
"Supdo", I said to my friend.
by freekynam October 11, 2011
 
1.
A combination of "whats up dog?" and "whats up though" making it both a freindly greeting and a question at the same time. Origins in Rochester and Detroit Michigan.
"so I walk up to my freind and im like 'Sup Do?'"
by Pete October 29, 2003