A fiend is a person or monster who has evil plans in their mind. They may try to steal your soul, or perhaps just want to hurt your feelings. Either way, fiends never mean you well.
The Old English root word for fiend translates to “an enemy, the devil, a demon”: three cruel characters that would delight in causing you harm. Watch out for fiends like them. A less wicked type of fiend is one who loves something so much that it’s scary, like a sugar-addicted jelly bean fiend, or a snowboarding fiend. Fiend is a good example of the “i before e except after c” rule, so follow that and you’ll always spell it right.