expression, noun
one's imagination; just one's mind playing tricks
Used when dismissing a feeling, suspicion, or perception as not real, often in the phrase 気のせいだ or 気のせいか.
誰かが呼んだ気がしたけど、気のせいだった。
I thought I heard someone call my name, but it was just my imagination.
気のせいか、少し寒くなった気がする。
Maybe it's just me, but it feels a little colder.
From 気 (mind, feeling) + の (possessive particle) + せい (fault, cause). Literally 'the fault of one's mind', implying that the mind is responsible for a mistaken impression.