In the 16th century, Copernicus (yes, the physician and astronomer) commanded the Allenstein castle in Prussia which was besieged by Teutonic Knights. A “plague” in the castle rendered many soldiers ill, including Copernicus. He divided the people into various diets and noted that the contagion was related to bread intake.
Soldiers atop the castle worked 12 hour shifts and took lunch while on duty (no workers’ rights there!). Copernicus determined that the bread-loaf was contaminated when accidentally dropped on the floor whilst being carried up the narrow steep stairs to the turrets by untrained staff pressed into waitering. The staff put the dropped bread back on the tray but as it was black in colour, the contamination with dirt and pathogens was not noticed.
A friend suggested coating the loaf with a light-coloured spread so they would detect if it had been dropped. They used churned cream and when eaters elected to clean or reject visibly dirtied bread, the epidemic halted. The practice was publicised by Copernicus’ colleague, BUTTENADT, secretary to the Apothecaries and Physicians Guild. The practice of BUTTENADTING later became popular with the people and the shortened form of the word became BUTTERING.
PS (TG). The epidemic would not have been THE plague (Yersinia pestis – spread by flea bites) but likely to have been any one of several diseases transmitted from rat faeces. Must be coincidence that the word “butter” is derived from the Greek Bouturon (“cow-cheese”). And for those whose bread always falls butter-side down – that’s fundamental physics and the height of your table!