Crank, lower quality methamphetamine not the higher quality crystam version but the stuff that is brown some times and burns like hell. There are 2 types of gack. One eye and two eye. One eye refers to the burning pain and tearing in the eye after snorting. Two eye gack makes both eyes burn and water from snorting it in one nostril.
"I did some gack last week and I still feel like shit!"
by PharmaPharmer December 30, 2005
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Where I come from, Gack always refered to the residue left over from smoking bongs or buckets.

Gack was scraped out of the cone or the bucket 'head' and ususally chucked in an ashtray. However, those on the dole and rather skanky used to smoke this gack by wiping it on a rizla.

They were known as gack heads. Nasty.
"Duuuude, take a lug on this..."
"Err, no fanx - it's full of gack!"
by Powdermonkey May 24, 2006
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Come on lets go and buy a big bag of gack
by tomo April 26, 2004
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v. - to steal, snatch, grab, or otherwise take something into one's possession
I was thirsty, so I gacked a Coke from his fridge.

If you like the ring, gack it when the clerk turns around.
by BlackArthur January 3, 2004
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The slimy substance (usually green) that was dumped on people in earyl Nickelodeon tv shows
"Thats the incorrect answer so you get drenched by a bucket of gack"
by Cane May 1, 2005
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Similar to a circus "geek", a "gack" is a circus performer who sucks farts out of chickens.
It was amazing how the circus gack was able to withdraw flatulence from the hens and roosters.
by J.J.W. November 22, 2004
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The browny black substance found inside a firing tube after a big mix has been smoked through it,It smells terrible and stains lips, clothing and fingers
John smoked a massive bong and got serious gack on his lip.
The bong was covered in gack
by erbivore198999 February 25, 2009
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