Think I learned it from Mark Twain's A Connecticut Yankee In King Arthur's Court where it appears with some frequency:

But come—never mind about that; let’s—have you got such a thing as a map of that region about you? Now a good map—”

“Is it peradventure that manner of thing which of late the unbelievers have brought from over the great seas, which, being boiled in oil, and an onion and salt added thereto, doth—”

> Peradventure is a good word. Do you have to be a lawyer to know it?

Agreed that it's a good word. I'd never heard or seen it myself until I was a mid-level partner at my then-law firm, and our senior partner used the phrase "beyond peradventure."