A preconceived idea is one you built before you had enough facts or experience — and then you walked in carrying it. The word almost always rides with idea, notion, or opinion. Teachers and scientists like it because it names a quiet kind of unfairness: you are not trying to be mean, but the picture in your head got there first, so new information has to fight it.

before you had enough facts or experience在你還沒有足夠的事實或經驗之前， A preconceived idea is one you built你就先建立起來的想法，就叫做preconceived idea(先入為主的想法) — and then you walked in carrying it——然後帶著它走進去。 The word almost always rides with idea, notion, or opinion這個字幾乎都是跟idea、notion、或opinion這幾個字搭在一起用。 Teachers and scientists like it老師和科學家喜歡用這個字 because it names a quiet kind of unfairness因為它指出一種不動聲色的不公平： you are not trying to be mean你不是故意要壞， but the picture in your head got there first但腦子裡的畫面已經先卡位了， so new information has to fight it所以新的資訊得跟它對抗。

Though actually, the word does not mean the idea is always wrong. It means it arrived too early. A doctor who has already decided “this is just a cold” may miss something more serious.

Though actually不過其實， the word does not mean the idea is always wrong這個字不代表那個想法一定是錯的。 It means it arrived too early它的意思是這個想法來得太早了。 A doctor who has already decided "this is just a cold"一個已經認定「這只是感冒」的醫生 may miss something more serious可能會漏掉更嚴重的問題。

It is a fairly formal adjective. In chat, people say “I already had it in my head that…”

It is a fairly formal adjective這是個相當正式的形容詞。 In chat在聊天的時候， people say "I already had it in my head that…"人們會說「我腦子裡早就認定…」。