Originated as a term used to describe pulling a wheely on ones bicycle but has since evolved into a commonly heard piece of street banter.
On seeing your friend on the other side of the street, or perhaps whilst passing in a car with the window rolled down as a friendly rousing gesture shout "Uuuuuuuuup bys!" with raising pitch on the 'up' and even higher pitch for the secondary 'bys'.
by anna apple October 12, 2003
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