Refers to "Garth Algar" from waynes world.
When someone does something so radical and awesume
Dude, that was so Garth Algar!
To drop feces (shit) from a person's anus into another person's mouth. Then the person who received the shit to rub it all over her face and put it in her nose and earholes.
Me and my girlfriend did a shitagonga! It was amazing!