This Irish origin phrase is most commonly spelled in all caps. It is the exclamation meant for the most badass party ever. Often said as an excitement booster for the party to come, or yelled at those who stand between you and the keg. Wyatt Montgomery, soon to be published poet from Asheville North Carolina, coined the phrase along with the aid of those of whom he parties with on a regular basis.
a: Dude, are you ready for the brutality that is to come tonight?
b: <epic glare>
a: Dude? C'mon man, let me hear it!
a+b: PARTYGODDAMNHELLYEAH!!!!!!!! WOOOOHOOO!!!! <body slam> (if necessary)