Word usage changes, evolves. A word or a different meaning of a word often starts as slang, then expands to become common usage. It's usually (but not always) apparent from context what the writer means. And, following the guidance of Strunk and White, don't use a fancy, uncommon, or potentially confusing word when plain words will do the job. Writing "he was at a loss" or "she didn't know what to do" is a few more words but much clearer to more readers than "he was nonplussed."