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1.
mouthing off, talking shit or just being loud.

Quick for running your yapper
Dicky - "What was that guy sayin about Celtics tickets back there"

Biff - "You mean that dude runnin yap at Billys? I don't know, alls I heard was blahblahblah n rarara"
by chrisking5150 December 02, 2010