The adverb erstwhile has been part of English since the 16th century, but it is formed from two words that are much older. It comes from the Old English words ær, meaning "early," and hwīl, which has much the same meaning as the modern word while. (The English word ere, meaning "before," is also descendant of ær.)

How do you use the word erstwhile?

You use erstwhile to describe someone that used to be the type of person indicated, but no longer is. He ran away with an erstwhile friend of his wife's.

What is an erstwhile friend?

Erstwhile comes from Old English and is a formal way to say "former." You can go to your class reunion 20 years after you graduate high school and see your erstwhile friends. That means that they had been your friends, but you lost touch with them, so that they weren't friends of yours anymore.