An attack usually consisting of 3 parts:
1.) you hit your friend...or foe...in the chest usually with the back of the hand.
2.) you then hit them in the bottom of the chin with your palm
3.) you say "sac" for the first part and "gouy!" for the second. saying this must be as loud and annoying as possible. Using any other two syllable word in place of sac-gouy works as well as the situation calls for it

Literally everyone, ever. Including the writer of this definition.