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West-Irish slang for 'Guinness Shite'- the black/green greasy monstrosity that exits the body after a night out drinkin' Guinness. Usually accompanied by an unholy smell that fills the entire room, leaving all others in the vicinity shouting or roaring, generally expressing extreme disgust and anger.
An accompaniment to the Slypack is a Packsly; this is a Guinness fart, which is oftentimes just as greasy as the Slypack itself and a whole lot more Pungent, akin to a a dead skunk crossed with... stale Guinness.

The Slypack is generally enjoyable to the creator, and for obvious reasons not so enjoyable to those who encounter it; therefore some view the Slypack as a practical joke under certain circumstances, i.e. when there's only one toilet in the house and, deviously, a Slypack blocks the pipe, not only disallowing other Slypacks to be made but also filling the house with it's own horrid odor.
"Christ almighty fellas, I'm after making the most unholy Slypack inside in the toilet,"

"Ah for fuck's sake, he's after making a Slypack again. That's the fifth time this morning,"

"I was up all night with a mad bout of the Packsly fellas- i thought it was the baked beans but then i remembered the feckin' Guinness,"
by Ryan Lavin April 27, 2008
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