When on uses the excuse of being an alcohlic to justify doing something incredibly stupid or unorthodox. The term was originally coined after Mel Gibson repeatedly used the excuse of being an alcoholic for doing stupid things; notably making the Passion of the Christ.
Jerrell: Yo, that guy just got raped by that Frat Boy 12 times in an hour!
Liam: Yeah, but he's got the Mel Gibson Defence.
Jerrell: True dat. Hey, you thinking what I'm thinking?