It was just a passing thought.
→ 그냥 지나가는 생각이었어.

[예문]

A: Did you say you wanted to get a nose job?
A: 너 코 성형하고 싶다고 했어?

B: I'm not sure. It was just a passing thought.
B: 잘 모르겠어. 그냥 지나가는 생각이었어.

A: I didn't know you believed in cosmetic surgery.
A: 네가 성형수술에 관심이 있는지 몰랐는데.

B: I didn't say I was going to get one.
    I'm just thinking about it.
B: 하겠다는 말이 아니라, 그냥 생각 한번 해보는거야.