Somehow it never occurred to me before (unless I've forgotten) to wonder about the etymology of the word "mile."

In Adam Roberts's novel On, there's a moment when a character learns the meaning of a foreign word, "mile," as something like "thousands of arm-lengths." (Don't have the book handy for exact quote.)

Which immediately (because of the interesting linguistic context in which that passage appeared) led me to wonder if "mile" was derived from "thousand" in the real world. And, sure enough, it's from Latin milia passuum, meaning "thousands of paces." Nifty.