Fais Gaffe: French Phrase Explained

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Expression: Fais gaffe !
Pronunciation: FAY GAFF
Meaning: Be careful! Watch out!
Literal translation: Make mistake!
Register: informal
Notes: The French expression fais gaffe is interesting because it means just the opposite of its literal translation. Faire une gaffe means "to make a mistake, to blunder," so you would think that "be careful!" would be something more like ne fais pas de gaffe! Apparently leaving out une is enough to turn the meaning around. You can also say Fais gaffe à toi to mean "Watch yourself."
Example
Ça peut être dangereux: Fais gaffe!

That can be dangerous: Be careful!