peradventure

peradventure \puhr-uhd-VEN-chuhr; pehr-\, adverb:

1. [Archaic] Possibly; perhaps.
2. Chance, uncertainty, or doubt.

It establishes beyond any peradventure of doubt that they were all wet and all wrong in their reports about the weapons of mass destruction, the chemical weapons, the biological weapons and the coming nuclear weapons as well.
-- Daniel Schorr, "interview Weekend Edition - Saturday, with Susan Stamberg", National Public Radio, July 10, 2004

The problem with Steve is that he looks like a liar. He is what a liar ought to look like. When he's telling God's own truth, hallelujah, you are certain beyond peradventure that he is lying.
-- "The journal of Lynton Charles", New Statesman, March 4, 2002

And he was, beyond peradventure, the greatest reforming Labour prime minister of the last century.
-- Peter Oborne, "Mr Blair has virtually unlimited power", Spectator, June 30, 2001

Peradventure derives from Old French per aventure, "by chance," from per, "through" (from Latin) + aventure, "chance," ultimately from the past participle of Latin advenire, "to arrive," from ad-, "to; toward" + venire, "to come."