Rap: A type of noise in which some rare examples sound a little like music.

It can be spelt Crap with a silent C.

Rap's origins have to do with "rapid speech". Urban males in New York City would stand on the street corner reciting their poetry at a fast delivery. Over time, they added music and/or beats whilst they rapped. There was a refinement of the style until you genuinely had the genre we hear to this day.