Someone who always defends and supports things that are clearly wrong and inexcusable. Examples of the sort of things that shit defenders will defend include corruption, injustice, unfairness, and general incompetence. They will often go out of their way to fabricate excuses for these things. Note that a shit defender does not necessarily always possess any of these negative traits themselves, they merely have to defend them.
Bob the shit defender can't stop making excuses for the War in Iraq, insisting that George W. Bush and his cabinet had every right reason to embark on this failed endeavor.
Rachel the shit defender constantly excuses her sociopath boyfriend's inane behavior, such as repeated drunk driving charges and general obnoxious and violently aggressive behavior, including unprovoked hostility toward random people.
Fred the shit defender insists that high school bullying is a normal and healthy process that bullying victims must go through. He claims that everyone who gets bullied deserves it in the first place, for not being popular, confident or strong enough, and that ultimately being bullied will make them stronger people, therefore making the act of bullying justified.