1) Drinking nothing but Guinness all night, nearly resulting in your death due to the nitrogen shits the next day.
2) drinking all the irish beer in a single bar
3) Consuming five or more Irish Car Bombs, in a single sitting.
Rost: Dude, you just destroyed the bathroom!
Rhett: Yeah, I committed Guinnesscide last night. Sorry
Bailey: Barkeep, I'll have a Smithwicks.
Barkeep: Sorry, but a bunch of kids came in here and committed Guinnesscide. But we have Bluemoon.
Bailey: Fuck Bluemoon
Rhett: When did Drew get so drunk?
Rost: He committed Guinnesscide a while ago.
Rhett: But he's drinking dirty Shirley Temples?
Rost: Yeah, but he had like ten Irish Car Bombs earlier.