“Fain” is an Old English word that doesn’t have a lot of modern context, but it is related to the verb “fawn.” They both come from the Germanic word “fægen,” meaning “to be happy or pleased.” Today, “fawning” is obsequious adoration, while “fain” describes a willingness or obligation. When “fain is used as an adjective, the willingness can come from opposite sides of the coin — someone feeling fain might be pleased as punch, or the activity might not be their cup of tea, but they’re obligated to do it anyway. As an adverb, “fain” means the activity is done gladly.