look up any word, like cunt:
 
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
 
1.
Methamphetamine (crank, speed). Describes the gag reflex triggered by the extreme burning sensation of the nasal passages when the drug is snorted, and the very bitter taste as it drains down the back of the throat.
The character 'Pooh-Bear' in the movie "The Salton Sea" had to have his nose surgically removed because he snorted too much gack.
by Anonymous July 13, 2003
 
2.
1.A term used for the Drug Crystal Meth Amphetamine

2A some what derogatory name for person under the influence of crystal meth.
1. "They just smoked a fat bowl of gack."

or Thats some killer Gack!"
2. "That guy is a total effin gack." or "Quit being such a gack, your going to get us busted!" or "Never bring that gack over to my house again!"
by L8Nighter March 02, 2011
 
3.
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
 
4.
The sound made by an individual who is in the process of sucking dick. Typically a gagging sound made from the throat.
Brenda gacks on that niggas dick because she has a terrible gag reflex.

That chicken head is always gacking choad in the park.
by Catsleeze September 23, 2013
 
5.
Any amount of useless or unwanted stuff (usually regarding possesions)
I've got too much damn gack in my attic. Guess I'll throw something on ebay.
by mudpup April 05, 2006
 
6.
Contraction for go back.
Would you like go g'ack home this weekend?
by Brian April 12, 2004
 
7.
To get messed up in the process of putting the team on your back and still going out like a champ.
His night would have been better if he hadn't Gacked after the race.
by bww13 December 05, 2010