preconceived idea
Noun
Meaning
an opinion formed beforehand without adequate evidence
Source: WordNetSynonyms
prejudice
preconception
prepossession
parti pris
presupposition
prejudgment
preconceived notion
preconceived opinion
Hypernyms
Examples
I had no preconceived idea what fame would be like, because I never thought I would be famous. I just wanted to do my work. Hell, I just wanted to pay my rent on time.
People in general have a preconceived idea of what prison is, from seeing documentaries or whatever.
he did not even try to confirm his preconceptions