fix idé

Etymology
From,.

Noun

 * 1) an idée fixe, an obsession, a compulsion (unreasonable idea or behavior that is stubbornly or obsessively adhered to)

Usage notes

 * More widely known in Swedish compared to English and not formal-sounding. Most native speakers are unlikely to be aware of the origin.
 * Plural forms like fixidéer might also be encountered, thinking of the term as a single word.