It would seem that the "Ordeal", the idea that boiling water or drowning could prove a man's guilt or innocence, was a Germanic custom brought to Gaul during the migrations. It was one of two methods to prove one's innocence...the other being oath taking. The ordeal won favor in the late Merovingian period as too many people were abusing the oath system.
"Yes myLord I promise I didn't do that! I double dog promise!"