expression
plans are but plans; the schedule is not decided
A humorous or resigned set phrase meaning that plans are tentative and subject to change. Often used when a schedule is still up in the air.
来月の旅行、予定は未定だけど、楽しみにしてる。
Next month's trip is still up in the air, but I'm looking forward to it.
会議の日程はまだ決まってないよ。予定は未定ってやつだね。
The meeting date hasn't been decided yet. It's a case of 'plans are but plans,' you know.
予定 means 'plan' or 'schedule,' but 予定は未定 turns that into a witty remark about how plans often remain tentative.
A wordplay combining 予定 (plan) and 未定 (undecided), linked by the shared sound てい. The phrase is a modern colloquialism with no deeper historical origin.