A fiend is someone who craves something so intensely they can't get enough of it. A chess fiend plays game after game, studying strategies late into the night. A cookie fiend always wants another cookie, even after eating three. When you call someone a math fiend or a reading fiend, you're saying they're passionate about it to an almost extreme degree.

The word carries a playful exaggeration. You're not really calling someone a monster, you're emphasizing how strong their desire is. A fiend for pizza means someone who absolutely loves pizza and gets excited whenever it's around.

In older stories, a fiend was a wicked supernatural creature. This older meaning survives in the word fiendish, which describes something devilishly clever or cruel: a fiendish puzzle is wickedly difficult, designed to frustrate even smart solvers.

When your friend says “I'm a trivia fiend,” they're using the word's intensity to show genuine enthusiasm. It's stronger than saying “I like trivia” but lighter than saying “I'm addicted.” The word captures that feeling when a hobby or interest grabs hold of you and won't let go.