1. Like being raped except with more emphasis
2. Getting your ass beat down really badly
Origins come from the perception that black men are the most endowed of all races and thus being raped by them tends to hurt more than being raped by any other race, generally and stereotypically speaking.
1. A girl from my school claims she got nigger raped by Leroy... if that's true, she'll be walking funny for a while.
2. Margarito just got his ass beat by Manny Pacquiao; he got nigger raped!