Rue The Day Meaning
(idiomatic) to seriously regret one's actions.
Example: 1987, Withnail and I:
I: I think there may be something living in there. I think there may be something alive.
Withnail: What d'you mean? A rat?
I: Itâ€™s possible. Itâ€™s possible.
Withnail: Then the fucker will rue the day!