Slap, once defined as "a blow made with an opened hand", has now become the newest addition to a long list of urban southern slang. According to southerners, "slap" is the new word for sex.
When the bass speakers/woofers in your car produce a loud booming sound that feels as if someone is beating on your chest, which can also be heard and felt outside of the vehicle the sound is coming from.
Dislaimer: Any loud excessive sound(s) emitting from a vehicle, especially in a residential area is concidered "Disturbing the peace." Disturbing the peace is considered a misdemeanor and is punishable by either a fine or brief term in jail.