Margen
When Pope Clement IV died in 1268, the cardinals gathered in Viterbo to choose a successor. The factions were so bitterly opposed that months went by without an agreement. Then years.
The people of Viterbo, tired of hosting that endless deadlock, took action: they locked the cardinals inside the papal palace. When even that didn't move things along, they cut their food down to bread and water. And since they still couldn't decide, they stripped the roof off the building and left them exposed to the elements, hoping heaven would inspire them faster.
It worked, eventually: the election took almost three years, the longest in history. The winner, Gregory X, took note and laid down strict rules for locking up future electors.
From that confinement 'with a key' — cum clave in Latin — comes the word we still use: conclave.