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.
Next month's trip is still up in the air, but I'm looking forward to it.
The meeting date hasn't been decided yet. It's a case of 'plans are but plans,' you know.
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.