Word commonly used in Hip Hop music during the 90's, most notably by American Underground/Indie hip hop artists, signifying gun shot/use of gun etc.
As well as this, it was used by bikers in the 80's as reference to the sound of their engines.
Now most notably used in the UK by chavs who get to excited by anything and everything.
1 - If you ever think of tryna play me then blam, brap brap
2 - this engine is one hell of a mother, braaaaaaaaaaap
3 - init bruv, brap!!