2)to boast to someone else.
Cara: Brrrrrap! Brap Brap Brap! yeh knew i cud win it!
Gameshow host: yeh well done cara!
2) Cara: hahahaha shame lisa i won and you didnt...brap! in yur face, brrrrapp!
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.
2 - this engine is one hell of a mother, braaaaaaaaaaap
3 - init bruv, brap!!