slap your neck
The act of a hard neck slap; if someone looks at a persons hand symbol below the waist or if someone does something idiotic.
Person 1: Hey whats 6 x 3?
Person 2: 54.
Person 1: It's 18 idiot. Slap your neck!
Person 2: 54.
Person 1: It's 18 idiot. Slap your neck!