Can be used synonymously with "to give someone shit," but in a less profane way. To Nag. Jewish children are often on the receiving end from their mothers.
"The guys gave me flak for my new sweater, they thought it was girly."
To knock someone unconscious very quickly.
"dayum bro, Chris got flakked"
To be so drunk that one no longer is capable of normal thought processes.
Why can’t he talk? “Bro he’s flakked”
In the Unreal series of fps games, the act of spraying Flak cannon, rocket launcher, Instagib, or Biorifle shots over a large object or past a blind corner in the hope that such fire will cause a frag. Usually practiced by newbs and llamas.
Player: FFS stop flakking down the main corridor! You're just blocking the main access to the ball room!