1: The sound a car trunk makes when playing a loud car stereo with huge, high-powered subwoofers in it.
2: Also can be used to describe the act of playing a song with loud bass.
"Blappin" is a slang word most commonly used in and around the California SF Bay Area. As part of the Bay Area's Hyphy Movement, "blappin" has been used in hip-hop songs by various Bay Area rappers such as Keak Da Sneak.
The word most likely evolved from the word "slappin" which also has the same definition in Bay Area slang.
"Damn that shit sounds clean. You can hear it blappin in da trunk from a mile away."
"What it do?"
"I'm just blappin dis new E-40 shit. What's good witchu?"