I have this happen to me at times ... dunno why ...
It means for a plan to go wrong or something to be broken! It is not good news if something's been knocked on the head.
Use in Normal SpeechEdit
When something's gone wrong.
"Oh pants! That's the trip knocked on the head - I've had a heart scare!"
When something just won't work.
"Boom. Innacurate nickname! That's knocked that on the head - unless ... "