Vorpal means “keen” (in the sense of “sharp”) or “deadly” and where it comes from is simple enough: it comes from the fertile and inventive imagination of Lewis Carroll. Vorpal is a word invented by Lewis Carroll in Alice Through the Looking Glass. The exact quote is: “The vorpal blade went snicker-snack!” The word has been picked up and used by others. For instance, Auden, in one of his poems, speaks of “the mechanised barbarian / The vorpal sword of an agrarian”. And The New Yorker once combined it in a sentence with another made-up Lewis Carroll word “frabjous” (meaning “fair” and “joyous”), writing: “A frabjous sort of place in a somewhat vorpal neighbourhood”. Meaning, I assume, a pleasant place in a dangerous (or even “deadly”) neighbourhood.