To purloin is to take something that belongs to someone else—that is, to steal it. Purloin is much more formal-sounding than steal, but is often—though not always—encountered in humorous contexts, suggesting that the theft is not serious.
The puppy managed to purloin a few cookies from the plate when no one was looking.
The studio stepped up security, fearing that someone might attempt to purloin a copy of the script for the show’s season finale.