The online Oxford English Dictionary has added a note to each entry showing how often it appears in current use. Peradventure appears in band 2, which the dictionary says contains “terms which are not part of normal discourse and would be unknown to most people.” The Times mocked Labour MP Harriet Harman in April 2015 for using it on a BBC television discussion programme (“if I make it absolutely clear, beyond peradventure ...”). The Times writer admitted he had to look it up.
Peradventure means “uncertainty” or “chance”. Beyond peradventure (sometimes as beyond a peradventure) is a fixed phrase that can pop up from the subconscious of a well-read but stressed person without allowing its owner time to think about whether it would be understood. It may be rendered in everyday English as “beyond question” or “without doubt”.
It may be adventurous to use it but where’s the adventure in it?
Historically, there is none. It comes to us from Old French per aventure, by chance. Aventure has had a mildly exotic history. We can trace it back to Latin adventura, a future form of the verb advenire, to happen — so something that may occur. By the time it reached Old French it could variously mean destiny or fate, a chance event, an accident, fortune or luck. The sense of aventure that was first taken into English was that of a chance event or accident.
The French word also came to be used in English as adventure, also at first for some chance event, but then for a risk of danger or loss. (Marine insurers still sometimes use adventure to mean the time during which insured goods are at risk.) Its sense shifted to a hazardous undertaking or audacious exploit — especially the sort carried out by medieval knights — but much more recently softened to sometimes mean merely a novel or exciting experience.