A quick strike to the top of the head with the hand. The clobber-knocker is applied by clenching the fist and swinging down on top of the head, using only the flat part of the fingers between the first and second knuckles. It is a very practical form of a sucker punch.

Brendan was pissing me off one day, so I gave him a clobber-knocker to show him how I felt.