What does “fais gaffe” mean in French?

Fais gaffe ! is the everyday informal way to say “careful”. The trap is the article. Faire gaffe, with nothing in front of gaffe, means to watch out and pay attention. Faire une gaffe means to put your foot in it — to say the wrong thing to the wrong person. One small word turns a warning into a confession, and the two are used constantly, so mishearing which one you just got told matters.

Faire attention is the neutral version that works in any register and with anyone: faites attention à la marche. Save fais gaffe for friends, and use faites attention with strangers or at work.