famous last words
US /ˈfeɪməs lɑst wɜrdz/
UK /ˈfeɪməs lɑst wɜrdz/

1.
a phrase used to suggest that someone's confident statement is likely to be proven wrong, often with humorous or ironic intent
:
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“I’ll just quickly finish this, no problem.” “Famous last words!”
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He said he'd never fall in love again. Famous last words, he's getting married next month!