15
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 01, 2005
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16
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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17
Used to describe female pyhsique. Particularly her stomach and breasts. When a female's stomach sticks out further then her breasts. aka (When her gut sticks out further then her rack)
She's got a Gack
When her gut sticks out further then her rack.
by Rob K July 24, 2006
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18
A really nasty substance. Like ear wax, saliva, boogers etc.
"Dude! Your ear phones are full of gack!"
by PaleteroJ March 27, 2010
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19
A word or sound muttered/or written by Professors of Anatomy and Physiology when proper terminlogy is not used. Also used un situations of extreme exacerbation.
Student Answer: Gluteus Med.
Professor Commentary on test: GACK! This is not proper terminology.

"GACK!, be quiet now! Or I will leave!"
by Rojay November 30, 2006
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20
When your girl...or guy(it is 2017)...gags on your dick
I don't care that Kaia is his girlfriend. Imma gack Dylan tonight at 12:04.
via giphy
by UpNextVanillaSwirlOnThePole March 09, 2017
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21
A noun used to insult anyone that looks, talks, or acts like a dork.
OMG, don't let Stewart come with us tonight, that dude is a fucking gack.
by Richard Huang November 06, 2004
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