I'm new at this, but I remember seeing an explanation of the origin of 'kibosh' a long time ago, I don't remember where, but it seemed at the time to be a fairly respectable source. Their explanation was that, way back when, in a law court, if the judge was about to pronounce a sentence of death, he ceremonially laid a square of black linen over his wig before he spoke. This was in the British Isles, could have been Ireland, Scotland, Wales, or England itself. That square of black linen was called the 'kibosh' or death cap.
Regards, Pearliemay