気のせいだよ。

English Translation

It’s just your imagination.

Should せい be thought of as meaning “the fault” of 気?Or, should 気のせい be thought of as a phrase of its own?

@ericaw
気のせい is a frequent expression but not an idiom. It’s just a literal combination of 気 + particle の + せい.

Examples of 〜のせい:

台風のせいでコンサートが中止になった = The concert was cancelled due to the typhoon.

お前のせいで、全員遅刻じゃないか! = All of us would be late because of you, stupid!

Examples of 気:

メアリーは細かいことを気にしすぎる性格だ = Mary worries too much about tedious stuff.

道を渡るときは車に気をつけましょう = You should be careful when you cross a street.

最近、トムから連絡がなかったので、気にかけていました。そうですか。入院してたのですね。= I was concerned about Tom not contacting me lately. Ah, he was hospitalized. Makes sense.

So, 気 of 気にする etc. basically means “to emotionally pay attention to”. It’s not so rational.

気のせいだよ, therefore, means “You had a wrong hunch. That is why you had a wrong idea.” For example, you “sensed” someone breaking into your house through a window. So you assumed it was a ghost. The idea is nonsense, so other people say to you that 気のせいだよ.