A person that will always convince you to party, especially if you're unmotivated or hungover.
Lyle: "I'm so shakey and bloated, I will never party again. I retire."
Sage: "Don't be a fag on Natturday... let's shotgun some Natty and start this game of wizard sticks."
Lyle: "You're such a partymonger! Where would I be without you? My Nattmare is over!"
The act of overzealously agreeing to do things with friends and acquaintances during a party or other social gathering despite the fact that ordinarily you would have no interest in doing so. Often followed by flaking out and not following through with the plan.
Able agreed to go to the Museum of Modern Art with Betty and Charles, but he was really just partyjoining.