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1.
A sub-genre of rap usually characterized by overly repetitive song structure, unintelligible lyrics, obnoxious melodies, and excessive posturing and label-flashing by the artist.

A classic example of this would be "Crank That" by Soulja Boy.
Nick: Could you distinguish a single English word in the song we just heard?
Mike: Hell no, but what can you expect? It's retard rap.
by Naju February 26, 2008
 
2.
A badass rap song by a certain Shadowpriest of Coldblooded on the Cairne server in World of Warcraft.
Yo Shawn, queue up that retard rap for the Maly video son!
by Grenshaw January 30, 2009
 
3.
Also known as ringtone rap. Mainstream rap music that's more about posturing and telling you to buy crap than it is about anything meaningful. The "gangsta" pose, combined with the barely coherent, often materialistic lyrics, makes the performers sound retarded.

Easily identified by repetitive choruses that take up most of the song, and the rapper's insistence that you buy some new fad object that you'll be embarrassed to own in five years (such as grills, or parachute pants).
Person #1: "Yo g-dizzle, I just picked up dat new Paul Wall CD!"

Person #2: "Get that retard rap shit the fuck outta my face!"
by The Reverend Soup January 16, 2008
 
4.
a really stupid ass freestyle
That wasn't tight nigga that was a stright up retard rap son
by Radio December 12, 2003