Rue the day
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A dramatic phrase with roots in Shakespeare and older English literature, meaning to bitterly regret. 'Rue' derives from Old English 'hreowan' (to make sorry). Frasier hurls this theatrical warning at Niles as he storms out to reconcile with Maris.
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Mentioned once in Look Before You Leap.
Look Before You Leap
Season 3 · Episode 16 · S03E16
“You will rue the day!”