Professor Commentary on test: GACK! This is not proper terminology.
"GACK!, be quiet now! Or I will leave!"
2A some what derogatory name for person under the influence of crystal meth.
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!"