A Quidditch foul committed by the Keeper, which consists of pushing any part of his or her body through the back of the goal hoop to prevent a score (QA6).
Flacking will have been one of the fouls committed during the first Quidditch World Cup match (QA6).
Commentary
Etymology
The word "flack" (English) means criticism and censure. There is also an American word "flacking" meaning publicising or promoting. The Quidditch foul gives us a completely new meaning.
From the Web
Writing by J.K. Rowling on Pottermore: History of the Quidditch World Cup
Pensieve (Comments)
Tags: brooms / broomsticks cheating fairness fouls match protection score strategy tactics team