A rather old slur directed at Irish people. In the late 1800s, the Irish were viewed as a secondary race by White Supremacists (see "Inbreds, Elite English") as were black people. (Who were conversely referred to as "Smoked Irish." I shit you not. People back then had ISSUES.)
These days, the terms has been embraced by Irish wannabe gangsters and thugs, et al, as a badge of honour. Look to House of Pain and Eminem as heroes.
Chap one: "Cracka mutherfucker! Get yo ass out of here!"
Chap two: "Sorry, my bad!"
Chap one: "Yo, you talk funny. Where you from?"
Chap two: "I'm Irish."
Chap one: "Sheeeet, you shoulda told me you was a nigga turned inside out! Gimme some love!"
when something is so awesome and mind blowing that no other words or phrase can describe it.
In a club
John- "Aren't these beats amazing"
Bill-"Yeah, my penis just turned into a giraffe"
At a party a guy throws and open beer bottle in the air, does and backflip and then catch's it without spilling any beer
Tom- "Holy shit that was amazing"
Mike- "my penis just turned into a giraffe"
Being in a harsh rear-end collision while driving a VW Beetle (or any other rear-engine car with no airbags) and having the steering column skewer the driver. The air-cooled engine catches fire, essentially turning the driver into a grilling rod on top of a campfire.